Exoplanet-hunting telescope to begin search for another Earth in 2026

Europe's next big space mission -- a telescope that will hunt for Earth-like rocky planets outside of our solar system -- is on course to launch at the end of 2026.

PLATO, or PLAnetary Transits and Oscillations of stars, is being built to find nearby potentially habitable worlds around Sun-like stars that we can examine in detail.

The space telescope will blast into orbit on Europe's new rocket, Ariane-6, which made its maiden flight last week after being developed at a cost of €4billion (£3.4billion).

Dr David Brown, of the University of Warwick, is giving an update on the mission at the Royal Astronomical Society's National Astronomy Meeting at the University of Hull this week.

"PLATO's goal is to search for exoplanets around stars similar to the Sun and at orbital periods long enough for them to be in the habitable zone," he said.

"One of the main mission objectives is to find another Earth-Sun equivalent pair, but it is also designed to carefully and precisely characterise the exoplanets that it finds (i.e. work out their masses, radii, and bulk density)."

PLATO isn't just an exoplanet hunter, however. It is also a stellar science mission.

As well as searching for exoplanets it will study the stars using a range of techniques including asteroseismology (measuring the vibrations and oscillations of stars) to work out their masses, radii, and ages.

Unlike most space telescopes, PLATO has multiple cameras -- including a UK-named one called ArthurEddington, after the famous astronomer and physicist who won the Royal Astronomical Society's Gold Medal in 1924.

It has 24 'Normal' cameras (N-CAMs) and 2 'Fast' cameras (F-CAMs). The N-CAMs are arranged into four groups of six cameras, with the cameras in each group pointing in the same direction but the groups slightly offset.

This gives PLATO a very large field of view, improved scientific performance, redundancy against failures, and a built-in way to identify 'false positive' signals that might mimic an exoplanet transit, Dr Brown explained.

"The planned observing strategy is to stare at two patches of sky, one in the North and one in the South, for two years each," he added.

"The Southern patch of sky has been chosen, while the Northern patch won't be confirmed for another few years."

Several of the spacecraft's components have finished their manufacturing programmes and are close to completing their calibration tests. This includes the UK-provided Front-End Electronics (FEE) for the N-CAMs.

Built by the Mullard Space Science Laboratory of University College London, these operate the cameras, digitise the images, and transfer them to the onboard data processing.

Ten of the final cameras have been built and tested and the first of these was mounted onto the optical bench -- the surface which keeps all cameras pointed in the right direction -- earlier this year.

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Newly discovered carbon monoxide-runaway gap can help identify habitable exoplanets

A carbon monoxide (CO)-runaway gap identified in the atmospheres of Earth-like planets by researchers at Tokyo Tech can help expand the search for habitable planets. This gap, identified through atmospheric modeling, is an indicator of a CO-rich atmosphere on Earth-like planets orbiting Sun-like stars. CO is an important compound for the formation of prebiotic organic compounds, which are building blocks for more complex molecules for the formation of life.

The search for habitable exoplanets involves looking for planets with similar conditions to the Earth, such as liquid water, a suitable temperature range and atmospheric conditions.

One crucial factor is the planet's position in the habitable zone, the region around a star where liquid water could potentially exist on the planet's surface.

NASA's Kepler telescope, launched in 2009, revealed that 20-50% of visible stars may host such habitable Earth-sized rocky planets.

However, the presence of liquid water alone does not guarantee a planet's habitability.

On Earth, carbon compounds such as carbon dioxide (CO2), methane (CH4), and carbon monoxide (CO) played a crucial role in shaping the climate and biogeochemistry and could have contributed to the emergence of life.

Taking this into consideration, a recent study by Associate Professor Kazumi Ozaki from Tokyo Institute of Technology, along with Associate Researcher Yasuto Watanabe from The University of Tokyo, aims to expand the search for habitable planets.

Published in the Astrophysical Journal on 10 January 2024, the researchers used atmospheric modeling to identify conditions that could result in a CO-rich atmosphere on Earth-like planets that orbit sun-like (F-, G-, and K-type) stars.

This phenomenon, known as CO runaway, is suggested by atmospheric models to have possibly occurred in early planetary atmospheres, potentially favoring the emergence of life.

"The possibility of CO runaway is critical in resolving the fundamental problem regarding the origin of life on Earth because various organic compounds suitable for the prebiotic chemistry are more likely to form in a CO-rich atmosphere than in a CO2-rich atmosphere," explains Dr. Ozaki.

The researchers modeled the CO cycle between the atmosphere and the oceans, considering the various sources of CO production, its transport mechanisms and the processes involved in its removal.

The photolysis of CO2, in which CO2 breaks down into CO when exposed to light, was considered the primary source of CO. Additional sources included photochemical reactions in the atmosphere, emissions from volcanic gases, and the hydrothermal decomposition of formaldehyde (H2CO) in the ocean.

The removal of CO from the atmosphere primarily occurred through its reaction with hydroxyl (OH) radicals formed due to the photolysis of water vapor, and to a lesser extent, by deposition to the planet's surface.

The researchers found that a CO runaway occurs when the CO production surpasses the removal by OH radicals.

This can occur due to higher CO2 levels or the presence of reducing gases from volcanoes which compete for the OH radicals.

At a temperature of 277 K, conditions for CO runaway are met when the partial pressure of CO2 exceeds 0.2 bar.

However, at higher temperatures (300 K), a CO runaway needs even higher CO2 and volcanic gas levels due to increased water vapor in the atmosphere, which is a major source of OH radicals.

Once initiated, the CO levels in the atmosphere are limited only by surface deposition, where CO is deposited onto the planet's surface.

Notably, the changes in the CO, CO2 and CH4 levels before and after the runaway effect led to a gap reflected in the phase space defined by the ratios of their partial pressures (pCO/pCO2 and pCH4/pCO2). "Our results suggest that this CO-runaway gap is a general feature of Earth-like lifeless planets orbiting Sun-like stars, providing insights into the characteristics and potential habitability of exoplanets," says Dr. Ozaki.

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The search for habitable planets expands

A University of Michigan astronomer and his team are suggesting a new way to expand the search for habitable planets that takes into account a zone not previously considered: the space between the star and what's called soot-line in planet-forming disks.

Worlds that form in this region -- a disk of dust rotating around a central star from which planets may be built -- could have surfaces rich in volatile carbon compounds quite different from Earth's. These planets would also be rich in organic carbon, but water poor, according toTed Bergin, who led the study that included geochemists, planetary scientists, astrochemists and exoplanet experts.

When we search for Earth-like planets, we are particularly interested not only in bodies that look like ours, but also in those that are formed by processes similar to ours. Current models of rocky exoplanets are built using Earth-like atmospheric conditions and bulk composition, including the molecules essential for life that form from carbon-based building blocks and water. These models also focus on zones within planet-forming disks called ice lines, regions distant enough from the disk's center star which mark where water or other key molecules transition from gas to solid phases.

Terrestrial worlds, like our planet, formed from solids. It has long been thought that Earth, which contains only approximately 0.1% water by mass, must have formed inside the water-ice line.

But that type of model may be too limited, Bergin said. To expand the search for habitable planets, Bergin and his research team suggest a new model that considers the soot line, a boundary closer to the solar system's star. Between this boundary and the star, organic compounds in solids sublimate out of the solid into gas. Considering this region would also encompass rocky planets that may have more carbon than Earth has, raising questions about what that means for habitability in these kinds of planets.

The findings by the interdisciplinary research team are published in Astrophysical Journal Letters.

"It adds a new dimension in our search for habitability. It may be a negative dimension or it may be a positive dimension," Bergin said. "It's exciting because it leads to all kinds of endless possibilities."

Just as Earth is poor in water, it is carbon poor as well, Bergin said. When forming, it likely received only 1 carbon atom per 100 available in planet-forming materials. Astronomers think the soot line explains why Earth has so little carbon. If Earth's building blocks formed inside the soot line, the temperature and solar radiation blasted the materials that would form the young planet, turning carbon-rich compounds into gas and limiting carbon in the solids that are supplied to the forming Earth.

The team's model theorizes about the formation of other planets born in between the soot line and water-ice lines.

Such a world does not appear to exist in our solar system, but our solar system is not representative of most known planetary systems around other stars, Bergin said. These other planetary systems look completely different. Their planets are closer to the sun and are much larger, ranging in size from what are called super-Earths to mini-Neptunes, he said.

"These are either big rocks or small gas giants -- that's the most common type of planetary system. So maybe, within all those other solar systems out in the Milky Way galaxy, there exists a population of bodies that we haven't recognized before that have much more carbon in their interiors. What are the consequences of that?" Bergin said. "What this means for habitability needs to be explored."

In their study, the team models what happens when a silicate-rich world with 0.1% and 1% carbon by mass and a variable water content forms in the soot line region. They found that such a planet would develop a methane-rich atmosphere through a process called outgassing. In this circumstance, organic compounds in a silicate-rich planet produce a methane-rich atmosphere.

The presence of methane provides a fertile environment for the generation of hazes through interactions with stellar photons. This is analogous to the generation of hazes from methane in Titan in our own solar system.

"Planets that are born within this region, which exists in every planet-forming disk system, will release more volatile carbon from their mantles," Bergin said. "This could readily lead to the natural production of hazes. Such hazes have been observed in the atmospheres of exoplanets and have the potential to change the calculus for what we consider habitable worlds."

Haze around a planet might be a signpost that the planet has volatile carbon in its mantle. And more carbon, the backbone of life, in the mantle of a planet means that the planet has a chance to be considered habitable -- or at least deserves a second glance, Bergin said.

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Does ice in the Universe contain the molecules making up the building blocks of life in planetary systems?

The James Webb Space Telescope -- the most precise telescope ever built -- was decisive in discovering the frozen forms of a long series of molecules, such as carbon dioxide, ammonia, methane, methanol and even more complex molecules, frozen out as ices on the surface of small dust grains.

The dust grains grow in size when being a part of the discs of gas and dust forming around young stars. This means that the researchers could study many of the molecules going into the forming of new exoplanets.

Researchers at the Niels Bohr Institute, University of Copenhagen, combined the discoveries from JWST with data from Atacama Large Millimeter Array (ALMA), making observations in other wavelengths than JWST and researchers from Aarhus University contributed with the necessary investigations in the laboratory.

"With the application of observations, e.g. from ALMA, it is possible for us to directly observe the dust grains themselves, and it is also possible to see the same molecules as in the gas observed in the ice" Lars Kristensen, associate Professor at the Niels Bohr Institute (NBI), explains.

"Using the combined data set gives us a unique insight into the complex interactions between gas, ice and dust in areas where stars and planets form" according to Jes Jørgensen, Professor at NBI.

"This way we can map the location of the molecules in the area both before and after they have been frozen out onto the dust grains and we can follow their path from the cold molecular cloud to the emerging planetary systems around young stars."

The content of ice in the molecular cloud was a decisive discovery

The ices were detected and measured by studying how starlight from beyond the molecular cloud was absorbed by icy molecules at specific infrared wavelengths visible to Webb.

This process leaves behind chemical fingerprints known as absorption spectra which can be compared with laboratory data to identify which ices are present in the molecular cloud.

In this study, the team targeted ices buried in a particularly cold, dense and difficult to investigate region of the Chamaeleon I molecular cloud, a region approximately 600 light-years from Earth which is currently in the process of forming dozens of young stars.

Along with star forming comes planet forming and the perspective for the researchers in the IceAge collaboration is basically to identify the role the ice plays in gathering the molecules necessary to form life.

"This study confirms that interstellar grains of dust are catalysts for the forming of complex molecules in the very diffuse gas in these clouds, something we see in the lab as well," Sergio Ioppolo explains, associate professor at Aarhus University, contributing with some of the experiments in the lab that were compared with the observations.

The sensitivity of JWST was an absolutely necessary precondition for the discovery

"We simply couldn't have observed these ices without Webb," elaborated Klaus Pontoppidan, JWST project scientist at the Space Telescope Science Institute, Baltimore, USA, who was involved in this research.

"The ices show up as dips against a continuum of background starlight. In regions that are this cold and dense, much of the light from the background star is blocked and Webb's exquisite sensitivity was necessary to detect the starlight and therefore identify the ices in the molecular cloud."

The IceAge team has already planned more observations with both Webb and other telescopes.

"These observations together with further laboratory studies will tell us which mixture of ices -- and therefore which elements -- can eventually be delivered to the surfaces of terrestrial exoplanets or incorporated into the atmospheres of giant gas or ice planets.

Read more at Science Daily

New small laser device can help detect signs of life on other planets

As space missions delve deeper into the outer solar system, the need for more compact, resource-conserving and accurate analytical tools has become increasingly critical -- especially as the hunt for extraterrestrial life and habitable planets or moons continues.

A University of Maryland-led team developed a new instrument specifically tailored to the needs of NASA space missions. Their mini laser-sourced analyzer is significantly smaller and more resource efficient than its predecessors -- all without compromising the quality of its ability to analyze planetary material samples and potential biological activity onsite. The team's paper on this new device was published in the journal Nature Astronomy on January 16, 2023.

Weighing only about 17 pounds, the instrument is a physically scaled-down combination of two important tools for detecting signs of life and identifying compositions of materials: a pulsed ultraviolet laser that removes small amounts of material from a planetary sample and an OrbitrapTM analyzer that delivers high-resolution data about the chemistry of the examined materials.

"The Orbitrap was originally built for commercial use," explained Ricardo Arevalo, lead author of the paper and an associate professor of geology at UMD. "You can find them in the labs of pharmaceutical, medical and proteomic industries. The one in my own lab is just under 400 pounds, so they're quite large, and it took us eight years to make a prototype that could be used efficiently in space -- significantly smaller and less resource-intensive, but still capable of cutting-edge science."

The team's new gadget shrinks down the original Orbitrap while pairing it with laser desorption mass spectrometry (LDMS) -- techniques that have yet to be applied in an extraterrestrial planetary environment. The new device boasts the same benefits as its larger predecessors but is streamlined for space exploration and onsite planetary material analysis, according to Arevalo.

Thanks to its diminutive mass and minimal power requirements, the mini Orbitrap LDMS instrument can be easily stowed away and maintained on space mission payloads. The instrument's analyses of a planetary surface or substance are also far less intrusive and thus much less likely to contaminate or damage a sample than many current methods that attempt to identify unknown compounds.

"The good thing about a laser source is that anything that can be ionized can be analyzed. If we shoot our laser beam at an ice sample, we should be able to characterize the composition of the ice and see biosignatures in it," Arevalo said. "This tool has such a high mass resolution and accuracy that any molecular or chemical structures in a sample become much more identifiable."

The laser component of the mini LDMS Orbitrap also allows researchers access to larger, more complex compounds that are more likely to be associated with biology. Smaller organic compounds like amino acids, for example, are more ambiguous signatures of life forms.

"Amino acids can be produced abiotically, meaning that they're not necessarily proof of life. Meteorites, many of which are chock full of amino acids, can crash onto a planet's surface and deliver abiotic organics to the surface," Arevalo said. "We know now that larger and more complex molecules, like proteins, are more likely to have been created by or associated with living systems. The laser lets us study larger and more complex organics that can reflect higher fidelity biosignatures than smaller, simpler compounds."

For Arevalo and his team, the mini LDMS Orbitrap will offer much-needed insight and flexibility for future ventures into the outer solar system, such as missions focused on life detection objectives (e.g., Enceladus Orbilander) and exploration of the lunar surface (e.g., the NASA Artemis Program). They hope to send their device into space and deploy it on a planetary target of interest within the next few years.

"I view this prototype as a pathfinder for other future LDMS and Orbitrap-based instruments," Arevalo said. "Our mini Orbitrap LDMS instrument has the potential to significantly enhance the way we currently study the geochemistry or astrobiology of a planetary surface."

Read more at Science Daily

Long-term liquid water also on non-Earth-like planets?

Liquid water is an important prerequisite for life to develop on a planet. As researchers from the University of Bern, the University of Zurich and the National Centre of Competence in Research (NCCR) PlanetS report in a new study, liquid water could also exist for billions of years on planets that are very different from Earth. This calls our currently Earth-centred idea of potentially habitable planets into question.

Life on Earth began in the oceans. In the search for life on other planets, the potential for liquid water is therefore a key ingredient. To find it, scientists have traditionally looked for planets similar to our own. Yet, long-term liquid water does not necessarily have to occur under similar circumstances as on Earth. Researchers of the University of Bern and the University of Zurich, who are members of the National Centre of Competence in Research (NCCR) PlanetS, report in a study published in the journal Nature Astronomy, that favourable conditions might even occur for billions of years on planets that barely resemble our home planet at all.

Primordial greenhouses

"One of the reasons that water can be liquid on Earth is its atmosphere," study co-author Ravit Helled, Professor of Theoretical Astrophysics at the University of Zurich and a member of the NCCR PlanetS explains. "With its natural greenhouse effect, it traps just the right amount of heat to create the right conditions for oceans, rivers and rain," says the researcher.

Earth's atmosphere used to be very different in its ancient history, however. "When the planet first formed out of cosmic gas and dust, it collected an atmosphere consisting mostly of Hydrogen and Helium -- a so-called primordial atmosphere," Helled points out. Over the course of its development, however, Earth lost this primordial atmosphere.

Other, more massive planets can collect much larger primordial atmospheres, which they can keep indefinitely in some cases. "Such massive primordial atmospheres can also induce a greenhouse effect -- much like Earth's atmosphere today. We therefore wanted to find out if these atmospheres can help to create the necessary conditions for liquid water," Helled says.

Liquid water for billions of years

To do so, the team thoroughly modelled countless planets and simulated their development over billions of years. They accounted not only for properties of the planets' atmospheres but also the intensity of the radiation of their respective stars as well as the planets' internal heat radiating outwards. While on Earth, this geothermal heat plays only a minor role for the conditions on the surface, it can contribute more significantly on planets with massive primordial atmospheres.

"What we found is that in many cases, primordial atmospheres were lost due to intense radiation from stars, especially on planets that are close to their star. But in the cases where the atmospheres remain, the right conditions for liquid water can occur," reports Marit Mol Lous, PhD student and lead-author of the study. According to the researcher at the University of Bern and the University of Zurich, "in cases where sufficient geothermal heat reaches the surface, radiation from a star like the Sun is not even necessary so that conditions prevail at the surface that allow the existence of liquid water."

"Perhaps most importantly, our results show that these conditions can persist for very long periods of time -- up to tens of billions of years," points out the researcher, who is also a member of the NCCR PlanetS.

Broadening the horizon for the search for extraterrestrial life

"To many, this may come as a surprise. Astronomers typically expect liquid water to occur in regions around stars that receive just the right amount of radiation: not too much, so that the water does not evaporate, and not too little, so that it does not all freeze," study co-author Christoph Mordasini, Professor of Theoretical Astrophysics at the University of Bern and member of the NCCR PlanetS explains.

"Since the availability of liquid water is a likely prerequisite for life, and life probably took many millions of years to emerge on Earth, this could greatly expand the horizon for the search for alien lifeforms. Based on our results, it could even emerge on so-called free-floating planets, that do not orbit around a star," Mordasini says.

Yet the researcher remains cautious: "While our results are exciting, they should be considered with a grain of salt. For such planets to have liquid water for a long time, they have to have the right amount of atmosphere. We do not know how common that is."

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Small stars share similar dynamics to our sun, key to planet habitability

Stars scattered throughout the cosmos look different, but they may be more alike than once thought, according to Rice University researchers.

New modeling work by Rice scientists shows that "cool" stars like the sun share the dynamic surface behaviors that influence their energetic and magnetic environments. This stellar magnetic activity is key to whether a given star hosts planets that could support life.

The work by Rice postdoctoral researcher Alison Farrish and astrophysicists David Alexander and Christopher Johns-Krull appears in a published study in The Astrophysical Journal. The research links the rotation of cool stars with the behavior of their surface magnetic flux, which in turn drives the star's coronal X-ray luminosity, in a way that could help predict how magnetic activity affects any exoplanets in their systems.

The study follows another led by Farrish and Alexander that showed a star's space "weather" may make planets in their "Goldilocks zone" uninhabitable.

"All stars spin down over their lifetimes as they shed angular momentum, and they get less active as a result," Farrish said. "We think the sun in the past was more active and that might have affected the early atmospheric chemistry of Earth. So thinking about how the higher energy emissions from stars change over long timescales is pretty important to exoplanet studies."

"More broadly, we're taking models that were developed for the sun and seeing how well they adapt to stars," said Johns-Krull.

The researchers set out to model what far-flung stars are like based on the limited data available. The spin and flux of some stars have been determined, along with their classification -- types F, G, K and M -- which gave information about their sizes and temperatures.

They compared the properties of the sun, a G-type star, through its Rossby number, a measure of stellar activity that combines its speed of rotation with its subsurface fluid flows that influence the distribution of magnetic flux on a star's surface, with what they knew of other cool stars. Their models suggest that each star's "space weather" works in much the same way, influencing conditions on their respective planets.

"The study suggests that stars -- at least cool stars -- are not too dissimilar from each other," Alexander said. "From our perspective, Alison's model can be applied without fear or favor when we look at exoplanets around M or F or K stars, as well, of course, as other G stars.

"It also suggests something much more interesting for established stellar physics, that the process by which a magnetic field is generated may be quite similar in all cool stars. That's a bit of a surprise," he said. This could include stars that, unlike the sun, are convective down to their cores.

"All stars like the sun fuse hydrogen and helium in their cores and that energy is first carried in the radiation of photons toward the surface," Johns-Krull said. "But it hits a zone about 60% to 70% of the way that's just too opaque, so it starts to undergo convection. Hot matter moves from below, the energy radiates away, and the cooler matter falls back down.

"But stars with less than a third of the mass of the sun don't have a radiative zone; they're convective everywhere," he said. "A lot of ideas about how stars generate a magnetic field rely on there being a boundary between the radiative and the convection zones, so you would expect stars that don't have that boundary to behave differently. This paper shows that in many ways, they behave just like the sun, once you adjust for their own peculiarities."

Farrish, who recently earned her doctorate at Rice and begins a postdoctoral research assignment at NASA's Goddard Space Flight Center soon, noted the model applies only to unsaturated stars.

"The most magnetically active stars are the ones we call 'saturated,'" Farrish said. "At a certain point, an increase in magnetic activity stops showing the associated increase in high energy X-ray emission. The reason that dumping more magnetism onto the star's surface doesn't give you more emission is still a mystery.

"Conversely, the sun is in the unsaturated regime, where we do see a correlation between magnetic activity and energetic emission," she said. "That happens at a more moderate activity level, and those stars are of interest because they might provide more hospitable environments for planets."

"The bottom line is the observations, which span four spectral types including both fully and partially convective stars, can be reasonably well represented by a model generated from the sun," Alexander said. "It also reinforces the idea that even though a star that is 30 times more active than the sun may not be a G-class star, it's still captured by the analysis that Alison has done."

"We do have to be clear that we're not simulating any specific star or system," he said. "We are saying that statistically, the magnetic behavior of a typical M star with a typical Rossby number behaves in a similar fashion to that of the sun which allows us to assess its potential impact on its planets."

A critical wild card is a star's activity cycle, which can't be incorporated into the models without years of observation. (The sun's cycle is 11 years, evidenced by sunspot activity when its magnetic field lines are most distorted.)

Johns-Krull said the model will still be useful in many ways. "One of my areas of interest is studying very young stars, many of which are, like low-mass stars, fully convective," he said. "Many of these have disc material around them and are still forming planets. How they interact is mediated, we think, by the stellar magnetic field.

"So, Alison's modeling work can be used to learn about the large-scale structure of very magnetically active stars, and that can then allow us to test some ideas about how these young stars and their disks interact."

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Earth-like biospheres on other planets may be rare

A new analysis of known exoplanets has revealed that Earth-like conditions on potentially habitable planets may be much rarer than previously thought. The work focuses on the conditions required for oxygen-based photosynthesis to develop on a planet, which would enable complex biospheres of the type found on Earth. The study is published today in Monthly Notices of the Royal Astronomical Society.

The number of confirmed planets in our own Milky Way galaxy now numbers into the thousands. However planets that are both Earth-like and in the habitable zone -- the region around a star where the temperature is just right for liquid water to exist on the surface -- are much less common.

At the moment, only a handful of such rocky and potentially habitable exoplanets are known. However the new research indicates that none of these has the theoretical conditions to sustain an Earth-like biosphere by means of 'oxygenic' photosynthesis -- the mechanism plants on Earth use to convert light and carbon dioxide into oxygen and nutrients.

Only one of those planets comes close to receiving the stellar radiation necessary to sustain a large biosphere: Kepler-442b, a rocky planet about twice the mass of the Earth, orbiting a moderately hot star around 1,200 light years away.

The study looked in detail at how much energy is received by a planet from its host star, and whether living organisms would be able to efficiently produce nutrients and molecular oxygen, both essential elements for complex life as we know it, via normal oxygenic photosynthesis.

By calculating the amount of photosynthetically active radiation (PAR) that a planet receives from its star, the team discovered that stars around half the temperature of our Sun cannot sustain Earth-like biospheres because they do not provide enough energy in the correct wavelength range. Oxygenic photosynthesis would still be possible, but such planets could not sustain a rich biosphere.

Planets around even cooler stars known as red dwarfs, which smoulder at roughly a third of our Sun's temperature, could not receive enough energy to even activate photosynthesis. Stars that are hotter than our Sun are much brighter, and emit up to ten times more radiation in the necessary range for effective photosynthesis than red dwarfs, however generally do not live long enough for complex life to evolve.

"Since red dwarfs are by far the most common type of star in our galaxy, this result indicates that Earth-like conditions on other planets may be much less common than we might hope," comments Prof. Giovanni Covone of the University of Naples, lead author of the study.

He adds: "This study puts strong constraints on the parameter space for complex life, so unfortunately it appears that the "sweet spot" for hosting a rich Earth-like biosphere is not so wide."

Read more at Science Daily

Raindrops also keep fallin' on exoplanets

One day, humankind may step foot on another habitable planet. That planet may look very different from Earth, but one thing will feel familiar -- the rain.

In a recent paper, Harvard researchers found that raindrops are remarkably similar across different planetary environments, even planets as drastically different as Earth and Jupiter. Understanding the behavior of raindrops on other planets is key to not only revealing the ancient climate on planets like Mars but identifying potentially habitable planets outside our solar system.

"The lifecycle of clouds is really important when we think about planet habitability," said Kaitlyn Loftus, a graduate student in the Department of Earth and Planetary Sciences and lead author of the paper. "But clouds and precipitation are really complicated and too complex to model completely. We're looking for simpler ways to understand how clouds evolve, and a first step is whether cloud droplets evaporate in the atmosphere or make it to the surface as rain."

"The humble raindrop is a vital component of the precipitation cycle for all planets," said Robin Wordsworth, Associate Professor of Environmental Science and Engineering at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) and senior author of the paper. "If we understand how individual raindrops behave, we can better represent rainfall in complex climate models."

An essential aspect of raindrop behavior, at least to climate modelers, is whether or not the raindrop makes it to the surface of the planet because water in the atmosphere plays a big role in planetary climate. To that end, size matters. Too big and the drop will break apart due to insufficient surface tension, regardless of whether it's water, methane or superheated, liquid iron as on an exoplanet called WASP-76b. Too small and the drop will evaporate before hitting the surface.

Loftus and Wordsworth identified a Goldilocks zone for raindrop size using just three properties: drop shape, falling speed, and evaporation speed.

Drop shapes are the same across different rain materials and primarily depend on how heavy the drop is. While many of us may picture a traditional tear-shaped droplet, raindrops are actually spherical when small, becoming squashed as they grow larger until they transition into a shape like the top of a hamburger bun. Falling speed depends on this shape as well as gravity and the thickness of the surrounding air.

Evaporation speed is more complicated, influenced by atmospheric composition, pressure, temperature, relative humidity and more.

By taking all of these properties into account, Loftus and Wordsworth found that across a wide range of planetary conditions, the math of raindrop falling means only a very small fraction of the possible drop sizes in a cloud can reach the surface.

"We can use this behavior to guide us as we model cloud cycles on exoplanets," said Loftus.

"The insights we gain from thinking about raindrops and clouds in diverse environments are key to understanding exoplanet habitability," said Wordsworth. "In the long term, they can also help us gain a deeper understanding of the climate of Earth itself."

Read more at Science Daily

On the quest for other Earths

In the search for planets capable of sustaining life, an international research team with members from ETH has taken a significant step forward. As the researchers reported recently in the journal Nature Communications, they found signs of a Neptune-sized planet in the Alpha Centauri star system, a mere 4.4 light years away from Earth. This exoplanet is located in a zone that may offer suitable conditions for life. The team was able to collect data with unprecedented sensitivity, thus registering even very weak signals.

Earth is a disruptive factor

Thanks to the new process, the researchers have advanced one step closer to a major goal of exoplanet research: the discovery of Earth-like planets capable of supporting life. Direct imaging of planets delivers information about the composition of their atmospheres and possibly even signs of life. To date, however, direct measurements have mostly found exoplanets that are larger than Jupiter and orbit far away from very young host stars. In other words, these planets fall outside the habitable zone where liquid water could form.

One reason that the search for Earth-like planets has so far proved fruitless is that it has been conducted in the near-infrared range, even though Earth-like planets that might have water are brightest in the mid-infrared range. Yet it is precisely in that range that measurements with normal telescopes are difficult, because that is where the Earth and its atmosphere are also at their brightest. This means the faint signals from exoplanets are lost in particularly strong background noise.

100 hours of observations

As reported in their study, the researchers have now been able to overcome this difficulty and take measurements in the mid-infrared range. They used the Very Large Telescope at the European Southern Observatory in Chile to examine Alpha Centauri stars A and B, logging nearly 100 hours over the course of a month. "Keeping the telescope pointed at the same star for such a long time is highly unusual," explains Anna Boehle, a postdoc in ETH Professor Sascha Quanz's group. As second author of the study, Boehle was heavily involved in evaluating the data. "We assessed more than five million images," she says.

To be able to detect the faint signals from potential planets, the researchers not only processed a huge volume of data, they also employed two sophisticated measurement techniques: one was to use a new deformable secondary telescope mirror, which made it possible to correct for distortions in the light coming through the Earth's atmosphere; and the other was to use a coronagraph to alternately block the light from each of the stars in turn at very short intervals. This let the scientists further reduce signal noise while examining the surroundings of both stars.

Signs of a planet

"Our findings indicate that in principle, this process enables us to discover smaller terrestrial planets capable of hosting life," Boehle explains, "and it represents a clear improvement over previous observation methods." Indeed, in their data the researchers found a light signal that may originate from a Neptune-sized planet. Boehle says, "Whether or not this signal is actually from a planet requires further study. To that end, we plan to combine the infrared measurements with other measurement methods."

From Science Daily

Chance played a major role in keeping Earth fit for life

 A study by the University of Southampton gives a new perspective on why our planet has managed to stay habitable for billions of years -- concluding it is almost certainly due, at least in part, to luck. The research suggests this may shorten the odds of finding life on so-called 'twin-Earths' in the Universe.

The research, published in the Nature journal Communications Earth & Environment, involved conducting the first ever simulation of climate evolution on thousands of randomly generated planets.

Geological data demonstrate that Earth's climate has remained continuously habitable for more than three billion years. However, it has been precariously balanced, with the potential to rapidly deteriorate to deep-frozen or intolerably hot conditions causing planet-wide sterility.

Professor Toby Tyrrell, a specialist in Earth System Science at the University of Southampton, explains: "A continuously stable and habitable climate on Earth is quite puzzling. Our neighbours, Mars and Venus, do not have habitable temperatures, even though Mars once did. Earth not only has a habitable temperature today, but has kept this at all times across three to four billion years -- an extraordinary span of geological time."

Many events can threaten the continuous stability of a planet -- asteroid impacts, solar flares and major geological events, such as eruptions of supervolcanoes. Indeed, an asteroid which hit the Earth 66 million years ago caused the extinction of more than 75 per cent of all species, killing off the dinosaurs along with many other species.

Previous computer modelling work on Earth habitability has involved modelling a single planet: Earth. But, inspired by discoveries of exoplanets (those outside of our solar system) that reveal that there are billions of Earth-like planets in our galaxy alone, a Southampton scientist took a novel approach to investigating a big question: what has led Earth to remain life-sustaining for so long?

To explore this, Professor Tyrrell tapped into the power of the University of Southampton's Iridis supercomputing facility to run simulations looking at how 100,000 randomly different planets responded to random climate-altering events spread out across three billion years, until they reached a point where they lost their habitability. Each planet was simulated 100 times, with different random events each time.

Having accrued a vast set of results, he then looked to see whether habitability persistence was restricted to just a few planets which were always capable of sustaining life for three billion years, or instead was spread around many different planets, each of which only sometimes stayed habitable for this period.

The results of the simulation were very clear. Most of those planets which remained life-sustaining throughout the three billion year period only had a probability, not a certainty, of staying habitable. Many instances were of planets which usually failed in the simulations and only occasionally remained habitable. Out of a total population of 100,000 planets, nine percent (8,700) were successful at least once -- of those, nearly all (about 8,000) were successful fewer than 50 times out of 100 and most (about 4,500) were successful fewer than 10 times out of 100.

The study results suggest chance is a major factor in determining whether planets, such as Earth, can continue to nurture life over billions of years. Professor Tyrrell concludes: "We can now understand that Earth stayed suitable for life for so long due, at least in part, to luck. For instance, if a slightly larger asteroid had hit Earth, or had done so at a different time, then Earth may have lost its habitability altogether.

"To put it another way, if an intelligent observer had been present on the early Earth as life first evolved, and was able to calculate the chances of the planet staying habitable for the next several billion years, the calculation may well have revealed very poor odds."

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About half of Sun-like stars could host rocky, potentially habitable planets

 

Exoplanets illustration

Since astronomers confirmed the presence of planets beyond our solar system, called exoplanets, humanity has wondered how many could harbor life. Now, we're one step closer to finding an answer. According to new research using data from NASA's retired planet-hunting mission, the Kepler space telescope, about half the stars similar in temperature to our Sun could have a rocky planet capable of supporting liquid water on its surface.

Our galaxy holds at least an estimated 300 million of these potentially habitable worlds, based on even the most conservative interpretation of the results in a new study to be published in The Astronomical Journal. Some of these exoplanets could even be our interstellar neighbors, with at least four potentially within 30 light-years of our Sun and the closest likely to be at most about 20 light-years from us. These are the minimum numbers of such planets based on the most conservative estimate that 7% of Sun-like stars host such worlds. However, at the average expected rate of 50%, there could be many more.

This research helps us understand the potential for these planets to have the elements to support life. This is an essential part of astrobiology, the study of life's origins and future in our universe.

The study is authored by NASA scientists who worked on the Kepler mission alongside collaborators from around the world. NASA retired the space telescope in 2018 after it ran out of fuel. Nine years of the telescope's observations revealed that there are billions of planets in our galaxy -- more planets than stars.

"Kepler already told us there were billions of planets, but now we know a good chunk of those planets might be rocky and habitable," said the lead author Steve Bryson, a researcher at NASA's Ames Research Center in California's Silicon Valley. "Though this result is far from a final value, and water on a planet's surface is only one of many factors to support life, it's extremely exciting that we calculated these worlds are this common with such high confidence and precision."

For the purposes of calculating this occurrence rate, the team looked at exoplanets between a radius of 0.5 and 1.5 times that of Earth's, narrowing in on planets that are most likely rocky. They also focused on stars similar to our Sun in age and temperature, plus or minus up to 1,500 degrees Fahrenheit.

That's a wide range of different stars, each with its own particular properties impacting whether the rocky planets in its orbit are capable of supporting liquid water. These complexities are partly why it is so difficult to calculate how many potentially habitable planets are out there, especially when even our most powerful telescopes can just barely detect these small planets. That's why the research team took a new approach.

Rethinking How to Identify Habitability

This new finding is a significant step forward in Kepler's original mission to understand how many potentially habitable worlds exist in our galaxy. Previous estimates of the frequency, also known as the occurrence rate, of such planets ignored the relationship between the star's temperature and the kinds of light given off by the star and absorbed by the planet.

The new analysis accounts for these relationships, and provides a more complete understanding of whether or not a given planet might be capable of supporting liquid water, and potentially life. That approach is made possible by combining Kepler's final dataset of planetary signals with data about each star's energy output from an extensive trove of data from the European Space Agency's Gaia mission.

"We always knew defining habitability simply in terms of a planet's physical distance from a star, so that it's not too hot or cold, left us making a lot of assumptions," said Ravi Kopparapu, an author on the paper and a scientist at NASA's Goddard Space Flight Center in Greenbelt, Maryland. "Gaia's data on stars allowed us to look at these planets and their stars in an entirely new way."

Gaia provided information about the amount of energy that falls on a planet from its host star based on a star's flux, or the total amount of energy that is emitted in a certain area over a certain time. This allowed the researchers to approach their analysis in a way that acknowledged the diversity of the stars and solar systems in our galaxy.

"Not every star is alike," said Kopparapu. "And neither is every planet."

Though the exact effect is still being researched, a planet's atmosphere figures into how much light is needed to allow liquid water on a planet's surface as well. Using a conservative estimate of the atmosphere's effect, the researchers estimated an occurrence rate of about 50% -- that is, about half of Sun-like stars have rocky planets capable of hosting liquid water on their surfaces. An alternative optimistic definition of the habitable zone estimates about 75%.

Kepler's Legacy Charts Future Research

This result builds upon a long legacy of work of analyzing Kepler data to obtain an occurrence rate and sets the stage for future exoplanet observations informed by how common we now expect these rocky, potentially habitable worlds to be. Future research will continue to refine the rate, informing the likelihood of finding these kinds of planets and feeding into plans for the next stages of exoplanet research, including future telescopes.

"Knowing how common different kinds of planets are is extremely valuable for the design of upcoming exoplanet-finding missions," said co-author Michelle Kunimoto, who worked on this paper after finishing her doctorate on exoplanet occurrence rates at the University of British Columbia, and recently joined the Transiting Exoplanet Survey Satellite, or TESS, team at the Massachusetts Institute of Technology in Cambridge, Massachusetts. "Surveys aimed at small, potentially habitable planets around Sun-like stars will depend on results like these to maximize their chance of success."

After revealing more than 2,800 confirmed planets outside our solar system, the data collected by the Kepler space telescope continues to yield important new discoveries about our place in the universe. Though Kepler's field of view covered only 0.25% of the sky, the area that would be covered by your hand if you held it up at arm's length towards the sky, its data has allowed scientists to extrapolate what the mission's data means for the rest of the galaxy. That work continues with TESS, NASA's current planet hunting telescope.

Read more at Science Daily

Some planets may be better for life than Earth

 Earth is not necessarily the best planet in the universe. Researchers have identified two dozen planets outside our solar system that may have conditions more suitable for life than our own. Some of these orbit stars that may be better than even our sun.

A study led by Washington State University scientist Dirk Schulze-Makuch recently published in the journal Astrobiology details characteristics of potential "superhabitable" planets, that include those that are older, a little larger, slightly warmer and possibly wetter than Earth. Life could also more easily thrive on planets that circle more slowly changing stars with longer lifespans than our sun.

The 24 top contenders for superhabitable planets are all more than 100 light years away, but Schulze-Makuch said the study could help focus future observation efforts, such as from NASA's James Web Space Telescope, the LUVIOR space observatory and the European Space Agency's PLATO space telescope.

"With the next space telescopes coming up, we will get more information, so it is important to select some targets," said Schulze-Makuch, a professor with WSU and the Technical University in Berlin. "We have to focus on certain planets that have the most promising conditions for complex life. However, we have to be careful to not get stuck looking for a second Earth because there could be planets that might be more suitable for life than ours."

For the study, Schulze-Makuch, a geobiologist with expertise in planetary habitability teamed up with astronomers Rene Heller of the Max Planck Institute for Solar System Research and Edward Guinan of Villanova University to identify superhabitability criteria and search among the 4,500 known exoplanets beyond our solar system for good candidates. Habitability does not mean these planets definitely have life, merely the conditions that would be conducive to life.

The researchers selected planet-star systems with probable terrestrial planets orbiting within the host star's liquid water habitable zone from the Kepler Object of Interest Exoplanet Archive of transiting exoplanets.

While the sun is the center of our solar system, it has a relatively short lifespan of less than 10 billion years. Since it took nearly 4 billion years before any form of complex life appeared on Earth, many similar stars to our sun, called G stars, might run out of fuel before complex life can develop.

In addition to looking at systems with cooler G stars, the researchers also looked at systems with K dwarf stars, which are somewhat cooler, less massive and less luminous than our sun. K stars have the advantage of long lifespans of 20 billion to 70 billion years. This would allow orbiting planets to be older as well as giving life more time to advance to the complexity currently found on Earth. However, to be habitable, planets should not be so old that they have exhausted their geothermal heat and lack protective geomagnetic fields. Earth is around 4.5 billion years old, but the researchers argue that the sweet spot for life is a planet that is between 5 billion to 8 billion years old.

Size and mass also matter. A planet that is 10% larger than the Earth should have more habitable land. One that is about 1.5 times Earth's mass would be expected to retain its interior heating through radioactive decay longer and would also have a stronger gravity to retain an atmosphere over a longer time period.

Water is key to life and the authors argue that a little more of it would help, especially in the form of moisture, clouds and humidity. A slightly overall warmer temperature, a mean surface temperature of about 5 degrees Celsius (or about 8 degrees Fahrenheit) greater than Earth, together with the additional moisture, would be also better for life. This warmth and moisture preference is seen on Earth with the greater biodiversity in tropical rain forests than in colder, drier areas.

Among the 24 top planet candidates none of them meet all the criteria for superhabitable planets, but one has four of the critical characteristics, making it possibly much more comfortable for life than our home planet.

Read more at Science Daily

Scientist searches for stellar phosphorus to find potentially habitable exoplanets

 A Southwest Research Institute scientist has identified stellar phosphorus as a probable marker in narrowing the search for life in the cosmos. She has developed techniques to identify stars likely to host exoplanets, based on the composition of stars known to have planets, and proposes that upcoming studies target stellar phosphorus to find systems with the greatest probability for hosting life as we know it.

"When searching for exoplanets and trying to see whether they are habitable, it's important that a planet be alive with active cycles, volcanoes and plate tectonics," said SwRI's Dr. Natalie Hinkel, a planetary astrophysicist and lead author of a new paper about this research in the Astrophysical Research Letters. "My coauthor, Dr. Hilairy Hartnett, is an oceanographer and pointed out that phosphorus is vital for all life on Earth. It is essential for the creation of DNA, cell membranes, bones and teeth in people and animals, and even the sea's microbiome of plankton."

Determining the elemental ratios for exoplanetary ecosystems is not yet possible, but it's generally assumed that planets have compositions similar to those of their host stars. Scientists can measure the abundance of elements in a star spectroscopically, studying how light interacts with the elements in a star's upper layers. Using these data, scientists can infer what a star's orbiting planets are made of, using stellar composition as a proxy for its planets.

On Earth, the key elements for biology are carbon, hydrogen, nitrogen, oxygen, phosphorus, and sulfur (or CHNOPS). In today's oceans, phosphorus is considered the ultimate limiting nutrient for life as it's the least available chemical necessary for biochemical reactions.

Hinkel used the Hypatia Catalog, a publicly available stellar database she developed, to assess and compare the carbon, nitrogen, silicon, and phosphorus abundance ratios of nearby stars with those in average marine plankton, the Earth's crust, as well as bulk silicate on Earth and Mars.

"But there's so little phosphorus stellar abundance data," Hinkel said. "Phosphorus data exists for only about 1% of stars. That makes it really difficult to figure out any clear trends in between the stars, let alone the role of phosphorus in the evolution of an exoplanet."

It's not that the stars are necessarily lacking phosphorus, but it's difficult to measure the element because it's detected in a region of the light spectrum not typically observed: at the edge of the optical (or visual) wavelengths of light and infrared light. Most spectroscopic studies are not tuned to find elements in that narrow range.

"Our Sun has relatively high phosphorus and Earth biology requires a small, but noticeable, amount of phosphorus," Hinkel continued. "So, on rocky planets that form around host stars with less phosphorus, it's likely that phosphorus will be unavailable for potential life on that planet's surface. Therefore, we urge the stellar abundance community to make phosphorus observations a priority in future studies and telescope designs."

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'Lost' world's rediscovery is step towards finding habitable planets

The rediscovery of a lost planet could pave the way for the detection of a world within the habitable 'Goldilocks zone' in a distant solar system.

The planet, the size and mass of Saturn with an orbit of thirty-five days, is among hundreds of 'lost' worlds that University of Warwick astronomers are pioneering a new method to track down and characterise in the hope of finding cooler planets like those in our solar system, and even potentially habitable planets.

Reported in Astrophysical Journal Letters, the planet named NGTS-11b orbits a star 620 light years away and is located five times closer to its sun than Earth is to our own.

The planet was originally found in a search for planets in 2018 by the Warwick-led team using data from NASA's TESS telescope. This uses the transit method to spot planets, scanning for the telltale dip in light from the star that indicates that an object has passed between the telescope and the star. However, TESS only scans most sections of the sky for 27 days. This means many of the longer period planets only transit once in the TESS data. And without a second observation the planet is effectively lost. The University of Warwick led team followed up one of these 'lost' planets using the telescopes at the Next-Generation Transit Survey (NGTS) in Chile and observed the star for seventy-nine nights, eventually catching the planet transiting for a second time nearly a year after the first detected transit.

Dr Samuel Gill from the Department of Physics at the University of Warwick said: "By chasing that second transit down we've found a longer period planet. It's the first of hopefully many such finds pushing to longer periods.

"These discoveries are rare but important, since they allow us to find longer period planets than other astronomers are finding. Longer period planets are cooler, more like the planets in our own Solar System.

"NGTS-11b has a temperature of only 160°C -- cooler than Mercury and Venus. Although this is still too hot to support life as we know it, it is closer to the Goldilocks zone than many previously discovered planets which typically have temperatures above 1000°C."

The Goldilocks zone refers to a range of orbits that would allow a planet or moon to support liquid water: too close to its star and it will be too hot, but too far away and it will be too cold.

Co-author Dr Daniel Bayliss from the University of Warwick said: "This planet is out at a thirty-five days orbit, which is a much longer period than we usually find them. It is exciting to see the Goldilocks zone within our sights."

Co-author Professor Pete Wheatley from the University of Warwick said: "The original transit appeared just once in the TESS data, and it was our team's painstaking detective work that allowed us to find it again a year later with NGTS.

"NGTS has twelve state-of-the-art telescopes, which means that we can monitor multiple stars for months on end, searching for lost planets. The dip in light from the transit is only 1% deep and occurs only once every 35 days, putting it out of reach of other telescopes. "

Read more at Science Daily