Astronomers discover first population of binary stripped stars

Astronomers at the University of Toronto have discovered a population of massive stars that have been stripped of their hydrogen envelopes by their companions in binary systems. The findings, published today in Science, shed light on the hot helium stars that are believed to be the origins of hydrogen-poor core-collapse supernovae and neutron star mergers.

For over a decade, scientists have theorized that approximately one in three massive stars are stripped of their hydrogen envelope in binary systems. Yet, until now, only one possible candidate had been identified.

"This was such a big, glaring hole," says co-lead author Maria Drout, an Assistant Professor in the David A. Dunlap Department of Astronomy & Astrophysics and a Dunlap Institute for Astronomy & Astrophysics Associate at the University of Toronto.

"If it turned out that these stars are rare, then our whole theoretical framework for all these different phenomena is wrong, with implications for supernovae, gravitational waves, and the light from distant galaxies," Drout says. "This finding shows these stars really do exist."

"Going forward, we are going to be able to do much more detailed physics with these stars," Drout says. "For example, predictions for how many neutron star mergers we should see are dependent on the properties of these stars, such as how much material comes off of them in stellar winds. Now, for the first time, we'll be able to measure that, whereas people have been extrapolating it before."

Binary stripped stars have been previously evoked to explain why a third of core-collapse supernovae contain much less hydrogen than a typical explosion of a Red Supergiant star. Drout and her colleagues propose that these newly discovered stars will eventually explode as hydrogen-poor supernovae. These star systems are also thought to be necessary to form neutron star mergers, like those that emit gravitational waves detected from Earth by the LIGO experiment.

In fact, the researchers believe that a few objects in their current sample are stripped stars with neutron star or blackhole companions. These objects are at the stage immediately before they become double neutron star or neutron star plus blackhole systems that could eventually merge.

"Many stars are part of a cosmic dance with a partner, orbiting each other in a binary system. They're not solitary giants but part of dynamic duos, interacting and influencing each other throughout their lifetimes," says Bethany Ludwig, a PhD student in in the David A. Dunlap Department of Astronomy & Astrophysics at the University Toronto and the third author on this paper. "Our work sheds light on these fascinating relationships, revealing a universe that is far more interconnected and active than we previously imagined."

"Just as humans are social beings, stars too, especially the massive ones, are rarely alone," Ludwig says.

As stars evolve and expand to become red giants, the hydrogen at the outer edges of one can be stripped by the gravitational pull of its companion -- leaving a very hot helium core exposed. The process can take tens of thousands, or even hundreds of thousands, of years.

Stripped stars are difficult to find because much of the light they emit is outside of the visible light spectrum and can be obstructed by dust in the universe or outshone by their companion stars.

Drout and her collaborators began their search in 2016. Having studied hydrogen-poor supernovae during her PhD, Drout set out to find the stripped stars thought to be at the heart of them during a NASA Hubble Postdoctoral Fellowship at the Observatories of the Carnegie Institution for Science. She met fellow co-author Ylva Götberg, now Assistant Professor at the Institute of Science and Technology Austria (ISTA), at a conference, who had recently built new theoretical models of what these stars should look like.

Drout, Götberg, and their collaborators designed a new survey to look in the ultraviolet part of the spectrum where extremely hot stars emit most of their light. While invisible to the naked eye, ultraviolet light can be detected by specialized instruments and telescopes.

Using data from the Swift Ultra-Violet/Optical Telescope, the researchers collected brightnesses for millions of stars in the Large and Small Magellanic Clouds, two of the closest galaxies to Earth. Ludwig developed the first wide-field UV catalog of the Magellanic Clouds and used UV photometry to detect systems with unusual UV emissions, signaling the possible presence of a stripped star.

They carried out a pilot study of 25 objects, obtaining optical spectroscopy with the Magellan Telescopes at Las Campanas Observatory between 2018 and 2022. They used these observations to demonstrate that the stars were hot, small, hydrogen-poor, and in binary systems -- all consistent with their model predictions.

Currently, the researchers are continuing to study the stars identified in this paper and expanding their search to find more. They will be looking both within nearby galaxies and within our own Milky Way with approved programs on the Hubble Space Telescope, the Chandra X-Ray Telescope, the Magellan Telescopes, and the Anglo-Australian Telescope. As part of this publication, all theoretical models and data used to identify these stars have been made public and available to other scientists.

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Reaching for the (invisible) stars

Supernovae-stellar explosions as bright as an entire galaxy-have fascinated us since time immemorial. Yet, there are more hydrogen-poor supernovae than astrophysicists can explain. Now, a new Assistant Professor at the Institute of Science and Technology Austria (ISTA) has played a pivotal role in identifying the missing precursor star population. The results, now published in Science, go back to a conversation the involved professors had many years ago as junior scientists.

Some stars do not simply die down, but explode in a stellar blast that could outshine entire galaxies. These cosmic phenomena, called supernovae, spread light, elements, energy, and radiation in space and send galactic shock waves that could compress gas clouds and generate new stars. In other words, supernovae shape our universe. Among these, hydrogen-poor supernovae from exploding massive stars have long puzzled astrophysicists. The reason: scientists have not been able to put their finger on their precursor stars. It is almost as if these supernovae appeared out of nowhere.

"There are many more hydrogen-poor supernovae than our current models can explain. Either we can't detect the stars that mature on this path, or we must revise all our models," says ISTA Assistant Professor Ylva Götberg. She pioneered this work together with Maria Drout, an Associated Faculty Member of the Dunlap Institute for Astronomy & Astrophysics, University of Toronto, Canada. "Single stars would typically explode as hydrogen-rich supernovae. Being hydrogen-poor indicates that the precursor star must have lost its thick hydrogen-rich envelope. This happens naturally in a third of all massive stars through envelope stripping by a binary companion star," says Götberg. Now, Götberg and Drout combined their areas of expertise in theoretical modeling and observation to hunt down the missing stars. Their quest is successful: they document a first-of-its-kind star population that finally bridges a large knowledge gap and sheds light on the origin of hydrogen-poor supernovae.

Binary stars and envelope stripping

The stars that Götberg and Drout search for go in pairs: interlocked in a binary star system. Some binary systems are well-known to us Earthlings: these include the brightest star in our night sky, Sirius A, and its faint companion star Sirius B. The Sirius binary system is located only 8.6 light-years away from Earth-a stone's throw in cosmic terms. This explains Sirius A's observed brightness in our night sky.

Astrophysicists expect the missing stars to be initially formed from massive binary systems. In a binary system, the stars would orbit around one another until the more massive star's thick, hydrogen-rich envelope expands. Eventually, the expanding envelope experiences a stronger gravitational pull to the companion star than to its own core. This causes a transfer of mass to begin, which eventually leads the entire hydrogen-rich envelope to be stripped off, leaving the hot and compact helium core exposed-more than 10 times hotter than the Sun's surface. This is precisely the type of stars that Götberg and Drout are looking for. "Intermediate mass helium stars stripped through binary interaction are predicted to play important roles in astrophysics. Yet, they were not observed until now," says Götberg. In fact, there is an important mass gap between the known classes of helium stars: the more massive Wolf-Rayet (WR) stars have more than 10 times the Sun's mass, and the low-mass subdwarf stars could have around half the Sun's mass. However, models have predicted the precursors of hydrogen-poor supernovae to lie between 2 and 8 solar masses following stripping.

Not just a needle in the haystack

Before Götberg and Drout's study, only one star was found to fulfill the expected mass and composition criteria and was called "Quasi-WR" (or "Almost Wolf-Rayet"). "Yet, the stars that follow this path have such a long lifetime that many must be scattered all over the observable universe," says Götberg. Did the scientists simply not "see" them? Thus, Götberg and Drout drew on their complementary expertise. With the help of UV photometry and optical spectroscopy, they identified a population of 25 stars that are consistent with the expectations for intermediate-mass helium stars. The stars are located in two well-studied neighboring galaxies, the Large and the Small Magellanic Clouds. "We showed that these stars were bluer than the stellar birthline, the bluest phase in a single star's lifetime. Single stars mature by evolving towards the redder region of the spectrum. A star only shifts in the opposite direction if its outer layers are removed-something that is expected to be common in interacting binary stars and rare among single massive stars," explains Götberg.

The scientists then verified their candidate star population using optical spectroscopy: they showed that the stars had strong spectral signatures of ionized helium. "Strong ionized helium lines tell us two important things: first, they confirm that the stars' outermost layers are dominated by helium and, second, that their surface is very hot. This is what happens to stars left as an exposed, compact, helium-rich core following stripping," says Götberg. Yet, both stars in a binary system contribute to the observed spectra. Thus, this technique allowed the researchers to classify their candidate population depending on which star contributed the most to the spectrum. "This work allowed us to find the missing population of intermediate-mass, stripped helium stars, the predicted progenitors of hydrogen-poor supernovae. These stars have always been there and there are probably many more out there. We must simply come up with ways to find them," says Götberg. "Our work may be one of the first attempts, but there should be other ways possible."

From graduate students at a conference to group leaders

The idea behind this project sparked in a discussion following a talk by Götberg at a conference that she and Drout attended during their graduate studies. Both scientists, then Early Career Researchers reaching for the stars, are now group leaders in their field. Götberg joined ISTA in September following her research at the Carnegie Observatories in Pasadena, California, as a NASA Hubble postdoctoral fellow. At ISTA, Götberg joins the Institute's growing ranks of young group leaders in astrophysics and leads her own group focused on studying the binary interactions of stars.

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'Triple star' discovery could revolutionize understanding of stellar evolution

A ground-breaking new discovery by University of Leeds scientists could transform the way astronomers understand some of the biggest and most common stars in the Universe.

Research by PhD student Jonathan Dodd and Professor René Oudmaijer, from the University's School of Physics and Astronomy, points to intriguing new evidence that massive Be stars -- until now mainly thought to exist in double stars -- could in fact be "triples."

The remarkable discovery could revolutionise our understanding of the objects -- a subset of B stars -- which are considered an important "test bed" for developing theories on how stars evolve more generally.

These Be stars are surrounded by a characteristic disc made of gas -- similar to the rings of Saturn in our own Solar System.

And although Be stars have been known for about 150 years -- having first been identified by renowned Italian astronomer Angelo Secchi in 1866 -- until now, no one has known how they were formed.

Consensus among astronomers so far has said the discs are formed by the rapid rotation of the Be stars, and that itself can be caused by the stars interacting with another star in a binary system.

Triple systems

Mr Dodd, corresponding author of the research, said: "The best point of reference for that is if you've watched Star Wars, there are planets where they have two Suns."

But now, by analysing data from the European Space Agency's Gaia satellite, the scientists say they have found evidence these stars actually exist in triple systems -- with three bodies interacting instead of just two.

Mr Dodd added: "We observed the way the stars move across the night sky, over longer periods like 10 years, and shorter periods of around six months. If a star moves in a straight line, we know there's just one star, but if there is more than one, we will see a slight wobble or, in the best case, a spiral.

"We applied this across the two groups of stars that we are looking at -- the B stars and the Be stars -- and what we found, confusingly, is that at first it looks like the Be stars have a lower rate of companions than the B stars. This is interesting because we'd expect them to have a higher rate."

However, Principal Investigator Prof Oudmaijer said: "The fact that we do not see them might be because they are now too faint to be detected."

Mass transfer

The researchers then looked at a different set of data, looking for companion stars that are further away, and found that at these larger separations the rate of companion stars is very similar between the B and Be stars.

From this, they were able to infer that in many cases a third star is coming into play, forcing the companion closer to the Be star -- close enough that mass can be transferred from one to the other and form the characteristic Be star disc.

This could also explain why we do not see these companions anymore; they have become too small and faint to be detected after the "vampire" Be star has sucked in so much of their mass.

The discovery could have huge impacts on other areas of astronomy -- including our understanding of black holes, neutron stars and gravitational wave sources.

Prof Oudmaijer said: "There's a revolution going on in physics at the moment around gravitational waves. We have only been observing these gravitational waves for a few years now, and these have been found to be due to merging black holes.

"We know that these enigmatic objects -- black holes and neutron stars -- exist, but we don't know much about the stars that would become them. Our findings provide a clue to understanding these gravitational wave sources."

He added: "Over the last decade or so, astronomers have found that binarity is an incredibly important element in stellar evolution. We are now moving more towards the idea it is even more complex than that and that triple stars need to be considered."

"Indeed," Oudmaijer said, "triples have become the new binaries."

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Most massive touching stars ever found will eventually collide as black holes

Two massive touching stars in a neighbouring galaxy are on course to become black holes that will eventually crash together, generating waves in the fabric of space-time, according to a new study by researchers at UCL (University College London) and the University of Potsdam.

The study, accepted for publication in the journal Astronomy & Astrophysics, looked at a known binary star (two stars orbiting around a mutual centre of gravity), analysing starlight obtained from a range of ground- and space-based telescopes.

The researchers found that the stars, located in a neighbouring dwarf galaxy called the Small Magellanic Cloud, are in partial contact and swapping material with each other, with one star currently "feeding" off the other. They orbit each other every three days and are the most massive touching stars (known as contact binaries) yet observed.

Comparing the results of their observations with theoretical models of binary stars' evolution, they found that, in the best-fit model, the star that is currently being fed on will become a black hole and will feed on its companion star. The surviving star will become a black hole shortly after.

These black holes will form in only a couple of million years, but will then orbit each other for billions of years before colliding with such force that they will generate gravitational waves -- ripples in the fabric of space-time -- that could theoretically be detected with instruments on Earth.

PhD student Matthew Rickard (UCL Physics & Astronomy), lead author of the study, said: "Thanks to gravitational wave detectors Virgo and LIGO, dozens of black hole mergers have been detected in the last few years. But so far we have yet to observe stars that are predicted to collapse into black holes of this size and merge in a time scale shorter than or even broadly comparable to the age of the universe.

"Our best-fit model suggests these stars will merge as black holes in 18 billion years. Finding stars on this evolutionary pathway so close to our Milky Way galaxy presents us with an excellent opportunity learn even more about how these black hole binaries form."

Co-author Daniel Pauli, a PhD student at the University of Potsdam, said: "This binary star is the most massive contact binary observed so far. The smaller, brighter, hotter star, 32 times the mass of the Sun, is currently losing mass to its bigger companion, which has 55 times our Sun's mass."

The black holes that astronomers see merge today formed billions of years ago, when the universe had lower levels of iron and other heavier elements. The proportion of these heavy elements has increased as the universe has aged and this makes black hole mergers less likely. This is because stars with a higher proportion of heavier elements have stronger winds and they blow themselves apart sooner.

The well-studied Small Magellanic Cloud, about 210,000 light years from Earth, has by a quirk of nature about a seventh of the iron and other heavy metal abundances of our own Milky Way galaxy. In this respect it mimics conditions in the universe's distant past. But unlike older, more distant galaxies, it is close enough for astronomers to measure the properties of individual and binary stars.

In their study, the researchers measured different bands of light coming from the binary star (spectroscopic analysis), using data obtained over multiple periods of time by instruments on NASA's Hubble Space Telescope (HST) and the Multi Unit Spectroscopic Explorer (MUSE) on ESO's Very Large Telescope in Chile, among other telescopes, in wavelengths ranging from ultraviolet to optical to near infrared.

With this data, the team were able to calculate the radial velocity of the stars -- that is, the movement they made towards or away from us -- as well as their masses, brightness, temperature and orbits. They then matched these parameters with the best-fit evolutionary model.

Their spectroscopic analysis indicated that much of the outer envelope of the smaller star had been stripped away by its larger companion. They also observed the radius of both stars exceeded their Roche lobe -- that is, the region around a star where material is gravitationally bound to that star -- confirming that some of the smaller star's material is overflowing and transferring to the companion star.

Talking through the future evolution of the stars, Rickard explained: "The smaller star will become a black hole first, in as little as 700,000 years, either through a spectacular explosion called a supernova or it may be so massive as to collapse into a black hole with no outward explosion.

"They will be uneasy neighbours for around three million years before the first black hole starts accreting mass from its companion, taking revenge on its companion."

Pauli, who conducted the modelling work, added: "After only 200,000 years, an instant in astronomical terms, the companion star will collapse into a black hole as well. These two massive stars will continue to orbit each other, going round and round every few days for billions of years.

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Ultracool dwarf binary stars break records

Northwestern University and the University of California San Diego (UC San Diego) astrophysicists have discovered the tightest ultracool dwarf binary system ever observed.

The two stars are so close that it takes them less than one Earth day to revolve around each other. In other words, each star's "year" lasts just 17 hours.

The newly discovered system, named LP 413-53AB, is composed of a pair of ultracool dwarfs, a class of very low-mass stars that are so cool that they emit their light primarily in the infrared, making them completely invisible to the human eye. They are nonetheless one of the most common types of stars in the universe.

Previously, astronomers had only detected three short-period ultracool dwarf binary systems, all of which are relatively young -- up to 40 million years old. LP 413-53AB is estimated to be billions of years old -- similar age to our sun -- but has an orbital period that is at least three times shorter than the all ultracool dwarf binaries discovered so far.

The research was published on March 1 in the Astrophysical Journal Letters.

"It's exciting to discover such an extreme system," said Chih-Chun "Dino" Hsu, a Northwestern astrophysicist who led the study. "In principle, we knew these systems should exist, but no such systems had been identified yet."

Hsu is a postdoctoral researcher in Northwestern's Center for Interdisciplinary Exploration and Research in Astrophysics(CIERA). He began this study while a Ph.D. student at UC San Diego, where he was advised by Professor Adam Burgasser.

The team first discovered the strange binary system while exploring archival data. Hsu developed an algorithm that can model a star based on its spectral data. By analyzing the spectrum of light emitted from a star, astrophysicists can determine the star's chemical composition, temperature, gravity and rotation. This analysis also shows the star's motion as it moves toward and away from the observer, known as radial velocity.

When examining the spectral data of LP 413-53AB, Hsu noticed something strange. Early observations caught the system when the stars were roughly aligned and their spectral lines overlapped, leading Hsu to believe it was just one star. But as the stars moved in their orbit, the spectral lines shifted in opposite directions, splitting into pairs in later spectral data. Hsu realized there were actually two stars locked into an incredibly tight binary.

Using powerful telescopes at the W.M. Keck Observatory, Hsu decided to observe the phenomenon for himself. On March 13, 2022, the team turned the telescopes toward the constellation Taurus, where the binary system is located, and observed it for two hours. Then, they followed up with more observations in July, October and December as well as January 2023.

"When we were making this measurement, we could see things changing over a couple of minutes of observation," Burgasser said. "Most binaries we follow have orbit periods of years. So, you get a measurement every few months. Then, after a while, you can piece together the puzzle. With this system, we could see the spectral lines moving apart in real time. It's amazing to see something happen in the universe on a human time scale."

The observations confirmed what Hsu's model predicted. The distance between the two stars is about 1% of the distance between the Earth and the sun. "This is remarkable, because when they were young, something like 1 million years old, these stars would have been on top of each other," said Burgasser.

The team speculates that the stars either migrated toward each other as they evolved, or they could have come together after the ejection of a third -- now lost -- stellar member. More observations are needed to test these ideas.

Hsu also said that by studying similar star systems researchers can learn more about potentially habitable planets beyond Earth. Ultracool dwarfs are much fainter and dimmer than the sun, so any worlds with liquid water on their surfaces -- a crucial ingredient to form and sustain life -- would need to be much closer to the star. However, for LP 413-53AB, the habitable zone distance happens to be the same as the stellar orbit, making it impossible to form habitable planets in this system.

"These ultracool dwarfs are neighbors of our sun," Hsu said. "To identify potentially habitable hosts, it's helpful to start with our nearby neighbors. But if close binaries are common among ultracool dwarfs, there may be few habitable worlds to be found."

To fully explore these scenarios, Hsu, Burgasser and their collaborators hope to pinpoint more ultracool dwarf binary systems to create a full data sample. New observational data could help strengthen theoretical models for binary-star formation and evolution. Until now, however, finding ultracool binary stars has remained a rare feat.

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Full 3-D view of binary star-planet system

By precisely tracing a small, almost imperceptible, wobble in a nearby star's motion through space, astronomers have discovered a Jupiter-like planet orbiting that star, which is one of a binary pair. Their work, using the National Science Foundation's Very Long Baseline Array (VLBA), produced the first-ever determination of the complete, 3-dimensional structure of the orbits of a binary pair of stars and a planet orbiting one of them. This achievement, the astronomers said, can provide valuable new insights on the process of planet formation.

Though more than 5,000 extrasolar planets have been discovered so far, only three have been discovered using the technique -- called astrometry -- that produced this discovery. However, the feat of determining the 3-D architecture of a binary-star system that includes a planet "cannot be achieved with other exoplanet discovery methods," said Salvador Curiel, of the National Autonomous University of Mexico (UNAM).

"Since most stars are in binary or multiple systems, being able to understand systems such as this one will help us understand planet formation in general," Curiel said.

The two stars, which together are called GJ 896AB, are about 20 light-years from Earth -- close neighbors by astronomical standards. They are red dwarf stars, the most common type in our Milky Way galaxy. The larger one, around which the planet orbits, has about 44 percent of the mass of our Sun, while the smaller is about 17 percent as massive as the Sun. They are separated by about the distance of Neptune from the Sun, and orbit each other once every 229 years.

For their study of GJ 896AB, the astronomers combined data from optical observations of the system made between 1941 and 2017 with data from VLBA observations between 2006 and 2011. They then made new VLBA observations in 2020. The continent-wide VLBA's supersharp resolution -- ability to see fine detail -- produced extremely precise measurements of the stars' positions over time. The astronomers performed extensive analysis of the data that revealed the stars' orbital motions as well as their common motion through space.

Detailed tracing of the larger star's motion showed a slight wobble that revealed the existence of the planet. The wobble is caused by the planet's gravitational effect on the star. The star and planet orbit a location between them that represents their common center of mass. When that location, called the barycenter, is sufficiently far from the star, the star's motion around it can be detectable.

The astronomers calculated that the planet has about twice the mass of Jupiter and orbits the star every 284 days. Its distance from the star is slightly less than Venus' distance from the Sun. The planet's orbit is inclined roughly 148 degrees from the orbits of the two stars.

"This means that the planet moves around the main star in the opposite direction to that of the secondary star around the main star," said Gisela Ortiz-León, of UNAM and the Max Planck Institute for Radioastronomy. "This is the first time that such dynamical structure has been observed in a planet associated with a compact binary system that presumably was formed in the same protoplanetary disk," she added.

"Additional detailed studies of this and similar systems can help us gain important insights into how planets are formed in binary systems. There are alternate theories for the formation mechanism, and more data can possibly indicate which is most likely," said Joel Sanchez-Bermudez, of UNAM. "In particular, current models indicate that such a large planet is very unlikely as a companion to such a small star, so maybe those models need to be adjusted," he added.

The astrometric technique will be a valuable tool for characterizing more planetary systems, the astronomers said. "We can do much more work like this with the planned Next Generation VLA (ngVLA)," said Amy Mioduszewski, of the National Radio Astronomy Observatory. "With it, we may be able to find planets as small as the Earth."

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Looking inside a neutron star -- new model will improve insights gleaned from gravitational waves

The unique oscillations in binary neutron stars right before they merge could have big implications for the insights scientists can glean from gravitational wave detection.

Researchers at the University of Birmingham have demonstrated the way in which these vibrations, caused by the interactions between the two stars' tidal fields as they get close together, affect gravitational-wave observations. The study is published in Physical Review Letters.

Taking these movements into account could make a huge difference to our understanding of the data taken by the Advanced LIGO and Virgo instruments, set up to detect gravitational waves -- ripples in time and space -- produced by the merging of black holes and neutron stars.

The researchers aim to have a new model ready for Advanced LIGO's next observing run and even more advanced models for the next generation of Advanced LIGO instruments, called A+, which are due to begin their first observing run in 2025.

Since the first gravitational waves were detected by the LIGO Scientific Collaboration and Virgo Collaboration in 2016, scientists have been focused on advancing their understanding of the massive collisions that produce these signals, including the physics of a neutron star at supra nuclear densities.

Dr Geraint Pratten, of the Institute for Gravitational Wave Astronomy at University of Birmingham, is lead-author on the paper. He said: "Scientists are now able to get lots of crucial information about neutron stars from the latest gravitational wave detections. Details such as the relationship between the star's mass and its radius, for example, provide crucial insight into fundamental physics behind neutron stars. If we neglect these additional effects, our understanding of the structure of the neutron star as a whole can become deeply biased."

Dr Patricia Schmidt, co-author on the paper and Associate Professor at the Institute for Gravitational Wave Astronomy, added: "These refinements are really important. Within single neutron stars we can start to understand what's happening deep inside the star's core, where matter exists at temperatures and densities we cannot produce in ground-based experiments. At this point we might start to see atoms interacting with each other in ways we have not yet seen -- potentially requiring new laws of physics."

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A new method to detect exoplanets

In recent years, a large number of exoplanets have been found around single 'normal' stars. New research shows that there may be exceptions to this trend. Researchers from The Autonomous University of Nuevo León (UANL), The National Autonomous University of Mexico (UNAM), and New York University Abu Dhabi suggest a new way of detecting dim bodies, including planets, orbiting exotic binary stars known as Cataclysmic Variables (CVs).

CVs are binary star systems in which the two stars are in extremely close proximity to each other; so close that the less massive object transfers mass to the more massive. CVs are typically formed of a small, cool type of star known as a red dwarf star, and a hot, dense star -- a white dwarf. Red dwarf stars have a mass between 0.07 and 0.30 solar masses and a radius of around 20% of the Sun's, while white dwarf stars have a typical mass of around 0.75 Solar masses and a very small radius similar to that of planet Earth.

In the CV system, the transfer of matter from the small star forms an accretion disk around the compact, more massive star. The brightness of a CV system mainly comes from this disk, and overpowers the light coming from the two stars. A third dim body orbiting a CV can influence the mass transfer rate between the two stars, and hence the brightness of the entire system. The method described in the new work is based on the change of brightness in the accretion disk due to perturbations of the third body that orbits around the inner two stars.

In their research, team leader Dr Carlos Chavez and his collaborators have estimated the mass and distance of a third body orbiting four different CVs using the changes in the brightness of each system. According to calculations carried out by the team, such brightness variations have very long periods in comparison to the orbital periods in the triple system. Two out of the four CVs appear to have bodies resembling planets in orbit around them.

Dr Chavez comments on the new findings, "Our work has proven that a third body can perturb a cataclysmic variable in such a way that can induce changes in brightness in the system. These perturbations can explain both the very long periods that have been observed -- between 42 and 265 days- and the amplitude of those changes in brightness." He adds, "Of the four systems we studied, our observations suggest that two of the four have objects of planetary mass in orbit around them."

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Planets of binary stars as possible homes for alien life

Nearly half of Sun-size stars are binary. According to University of Copenhagen research, planetary systems around binary stars may be very different from those around single stars. This points to new targets in the search for extraterrestrial life forms.

Since the only known planet with life, the Earth, orbits the Sun, planetary systems around stars of similar size are obvious targets for astronomers trying to locate extraterrestrial life. Nearly every second star in that category is a binary star. A new result from research at University of Copenhagen indicate that planetary systems are formed in a very different way around binary stars than around single stars such as the Sun.

"The result is exciting since the search for extraterrestrial life will be equipped with several new, extremely powerful instruments within the coming years. This enhances the significance of understanding how planets are formed around different types of stars. Such results may pinpoint places which would be especially interesting to probe for the existence of life," says Professor Jes Kristian Jørgensen, Niels Bohr Institute, University of Copenhagen, heading the project.

The results from the project, which also has participation of astronomers from Taiwan and USA, are published in the journal Nature.

Bursts shape the planetary system

The new discovery has been made based on observations made by the ALMA telescopes in Chile of a young binary star about 1,000 lightyears from Earth. The binary star system, NGC 1333-IRAS2A, is surrounded by a disc consisting of gas and dust. The observations can only provide researchers with a snapshot from a point in the evolution of the binary star system. However, the team has complemented the observations with computer simulations reaching both backwards and forwards in time.

"The observations allow us to zoom in on the stars and study how dust and gas move towards the disc. The simulations will tell us which physics are at play, and how the stars have evolved up till the snapshot we observe, and their future evolution," explains Postdoc Rajika L. Kuruwita, Niels Bohr Institute, second author of the Nature article.

Notably, the movement of gas and dust does not follow a continuous pattern. At some points in time -- typically for relatively shorts periods of ten to one hundred years every thousand years -- the movement becomes very strong. The binary star becomes ten to one hundred times brighter, until it returns to its regular state.

Presumably, the cyclic pattern can be explained by the duality of the binary star. The two stars encircle each other, and at given intervals their joint gravity will affect the surrounding gas and dust disc in a way which causes huge amounts of material to fall towards the star.

"The falling material will trigger a significant heating. The heat will make the star much brighter than usual," says Rajika L. Kuruwita, adding:

"These bursts will tear the gas and dust disc apart. While the disc will build up again, the bursts may still influence the structure of the later planetary system."

Comets carry building blocks for life

The observed stellar system is still too young for planets to have formed. The team hopes to obtain more observational time at ALMA, allowing to investigate the formation of planetary systems.

Not only planets but also comets will be in focus:

"Comets are likely to play a key role in creating possibilities for life to evolve. Comets often have a high content of ice with presence of organic molecules. It can well be imagined that the organic molecules are preserved in comets during epochs where a planet is barren, and that later comet impacts will introduce the molecules to the planet's surface," says Jes Kristian Jørgensen.

Understanding the role of the bursts is important in this context:

"The heating caused by the bursts will trigger evaporation of dust grains and the ice surrounding them. This may alter the chemical composition of the material from which planets are formed."

Thus, chemistry is a part of the research scope:

"The wavelengths covered by ALMA allow us to see quite complex organic molecules, so molecules with 9-12 atoms and containing carbon. Such molecules can be building blocks for more complex molecules which are key to life as we know it. For example, amino acids which have been fund in comets."

Powerful tools join the search for life in space

ALMA (Atacama Large Millimeter/submillimeter Array) is not a single instrument but 66 telescopes operating in coordination. This allows for a much better resolution than could have been obtained by a single telescope.

Very soon the new James Webb Space Telescope (JWST) will join the search for extraterrestrial life. Near the end of the decade, JWST will be complemented by the ELT (European Large Telescope) and the extremely powerful SKA (Square Kilometer Array) both planned to begin observing in 2027. The ELT will with its 39-meter mirror be the biggest optical telescope in the world and will be poised to observe the atmospheric conditions of exoplanets (planets outside the Solar System, ed.). SKA will consist of thousands of telescopes in South Africa and in Australia working in coordination and will have longer wavelengths than ALMA.

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ALMA animation of circling twin young stars

Researchers analyzed the accumulated data from the Atacama Large Millimeter/submillimeter Array (ALMA) and depicted the motion of a young twin star system XZ Tauri over three years. This first-ever "ALMA Animation" of twin stars sheds new light on the origins of the binary stars and the planets to be formed around them.

"This achievement was made possible by the high resolution and rich archival data of ALMA," says Takanori Ichikawa, the first author of the research paper and a former graduate student at Kagoshima University, Japan. "This research utilized three years of observation data. The results show the feasibility of a new research method using radio astronomical animations instead of conventional images. I hope that this method will help to clarify various astronomical phenomena in the future."

The Sun is a single star, but the Universe is full of binary stars, which are two stars orbiting each other. During its youth, each young star in a binary system is surrounded by a protoplanetary disk composed of molecular gas and dust. This disk is known to be the site of planet formation. Many planets associated with binary stars have actually been detected, but how the disks are formed in binary star systems and how planets are formed in those systems is still a mystery.

"In order to study the formation of planets in binary star systems, it is important to accurately determine the orbital motion of the two stars and the tilt of the individual protoplanetary disks," explains Shigehisa Takakuwa, a professor at Kagoshima University.

Researchers have suggested two formation mechanisms for binary systems; one is a breakup of a single large gaseous disk, and the other is fragmentation of the larger molecular cloud due to violent turbulence. In the former case, astronomers suppose that the orbit of the binary stars and the individual disks should be on the same plane. On the other hand, in the latter case, the orbital plane of the binary stars and the plane of the disks are expected to be different. This is a major issue that will affect the final orbits of the planets in binary systems.

The research team dug into the ALMA data archive and obtained the data for the young XZ Tau system taken in 2015, 2016, and 2017. They carefully analyzed the data and for the first time made an animation of the orbital motion of the binary stars, which shows that XZ Tau B moved 3.4 astronomical units (3.4 times the radius of Earth's orbit) around XZ Tau A during these three years.

The team figured out the three-dimensional structure of the orbit. In addition, analyzing the doppler effect and the distribution of the radio waves from the disk around each star in the XZ Tau system, they found that those disks are significantly misaligned with respect to each other, and also not in the same plane as the binary orbit.

Previous observations with ALMA had found examples of young binary stars with protoplanetary disks tilted with respect to each other (Note). However, this is the first time that the orbital motion of a binary system has been clarified, showing that the inclination is different from those of the circumstellar disks. These results support the idea that the XZ Tau system was formed via molecular cloud fragmentation.

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Are we missing other Earths?

Some exoplanet searches could be missing nearly half of the Earth-sized planets around other stars. New findings from a team using the international Gemini Observatory and the WIYN 3.5-meter Telescope at Kitt Peak National Observatory suggest that Earth-sized worlds could be lurking undiscovered in binary star systems, hidden in the glare of their parent stars. As roughly half of all stars are in binary systems, this means that astronomers could be missing many Earth-sized worlds.

Earth-sized planets may be much more common than previously realized. Astronomers working at NASA Ames Research Center have used the twin telescopes of the international Gemini Observatory, a Program of NSF's NOIRLab, to determine that many planet-hosting stars identified by NASA's TESS exoplanet-hunting mission are actually pairs of stars -- known as binary stars -- where the planets orbit one of the stars in the pair. After examining these binary stars, the team has concluded that Earth-sized planets in many two-star systems might be going unnoticed by transit searches like TESS's, which look for changes in the light from a star when a planet passes in front of it. The light from the second star makes it more difficult to detect the changes in the host star's light when the planet transits.

The team started out by trying to determine whether some of the exoplanet host stars identified with TESS were actually unknown binary stars. Physical pairs of stars that are close together can be mistaken for single stars unless they are observed at extremely high resolution. So the team turned to both Gemini telescopes to inspect a sample of exoplanet host stars in painstaking detail. Using a technique called speckle imaging, the astronomers set out to see whether they could spot undiscovered stellar companions.

Using the `Alopeke and Zorro instruments on the Gemini North and South telescopes in Chile and Hawai'i, respectively, the team observed hundreds of nearby stars that TESS had identified as potential exoplanet hosts. They discovered that 73 of these stars are really binary star systems that had appeared as single points of light until observed at higher resolution with Gemini. "With the Gemini Observatory's 8.1-meter telescopes, we obtained extremely high-resolution images of exoplanet host stars and detected stellar companions at very small separations," said Katie Lester of NASA's Ames Research Center, who led this work.

Lester's team also studied an additional 18 binary stars previously found among the TESS exoplanet hosts using the NN-EXPLORE Exoplanet and Stellar Speckle Imager (NESSI) on the WIYN 3.5-meter Telescope at Kitt Peak National Observatory, also a Program of NSF's NOIRLab.

After identifying the binary stars, the team compared the sizes of the detected planets in the binary star systems to those in single-star systems. They realized that the TESS spacecraft found both large and small exoplanets orbiting single stars, but only large planets in binary systems.

These results imply that a population of Earth-sized planets could be lurking in binary systems and going undetected using the transit method employed by TESS and many other planet-hunting telescopes. Some scientists had suspected that transit searches might be missing small planets in binary systems, but the new study provides observational support to back it up and shows which sizes of exoplanets are affected.

"We have shown that it is more difficult to find Earth-sized planets in binary systems because small planets get lost in the glare of their two parent stars," Lester stated. "Their transits are 'filled in' by the light from the companion star," added Steve Howell of NASA's Ames Research Center, who leads the speckle imaging effort and was involved in this research.

"Since roughly 50% of stars are in binary systems, we could be missing the discovery of -- and the chance to study -- a lot of Earth-like planets," Lester concluded.

The possibility of these missing worlds means that astronomers will need to use a variety of observational techniques before concluding that a given binary star system has no Earth-like planets. "Astronomers need to know whether a star is single or binary before they claim that no small planets exist in that system," explained Lester. "If it's single, then you could say that no small planets exist. But if the host is in a binary, you wouldn't know whether a small planet is hidden by the companion star or does not exist at all. You would need more observations with a different technique to figure that out."

As part of their study, Lester and her colleagues also analyzed how far apart the stars are in the binary systems where TESS had detected large planets. The team found that the stars in the exoplanet-hosting pairs were typically farther apart than binary stars not known to have planets. This could suggest that planets do not form around stars that have close stellar companions.

"This speckle imaging survey illustrates the critical need for NSF telescope facilities to characterize newly discovered planetary systems and develop our understanding of planetary populations," said National Science Foundation Division of Astronomical Sciences Program Officer Martin Still.

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The strange orbits of 'Tatooine' planetary disks

ALMA radio antennas at night.

Astronomers using the Atacama Large Millimeter/submillimeter Array (ALMA) have found striking orbital geometries in protoplanetary disks around binary stars. While disks orbiting the most compact binary star systems share very nearly the same plane, disks encircling wide binaries have orbital planes that are severely tilted. These systems can teach us about planet formation in complex environments.

In the last two decades, thousands of planets have been found orbiting stars other than our Sun. Some of these planets orbit two stars, just like Luke Skywalker's home Tatooine. Planets are born in protoplanetary disks -- we now have wonderful observations of these thanks to ALMA -- but most of the disks studied so far orbit single stars. 'Tatooine' exoplanets form in disks around binary stars, so-called circumbinary disks.

Studying the birthplaces of 'Tatooine' planets provides a unique opportunity to learn about how planets form in different environments. Astronomers already know that the orbits of binary stars can warp and tilt the disk around them, resulting in a circumbinary disk misaligned relative to the orbital plane of its host stars. For example, in a 2019 study led by Grant Kennedy of the University of Warwick, UK, ALMA found a striking circumbinary disk in a polar configuration.

"With our study, we wanted to learn more about the typical geometries of circumbinary disks," said astronomer Ian Czekala of the University of California at Berkeley. Czekala and his team used ALMA data to determine the degree of alignment of nineteen protoplanetary disks around binary stars. "The high resolution ALMA data was critical for studying some of the smallest and faintest circumbinary disks yet," said Czekala.

The astronomers compared the ALMA data of the circumbinary disks with the dozen 'Tatooine' planets that have been found with the Kepler space telescope. To their surprise, the team found that the degree to which binary stars and their circumbinary disks are misaligned is strongly dependent on the orbital period of the host stars. The shorter the orbital period of the binary star, the more likely it is to host a disk in line with its orbit. However, binaries with periods longer than a month typically host misaligned disks.

"We see a clear overlap between the small disks, orbiting compact binaries, and the circumbinary planets found with the Kepler mission," Czekala said. Because the primary Kepler mission lasted 4 years, astronomers were only able to discover planets around binary stars that orbit each other in fewer than 40 days. And all of these planets were aligned with their host star orbits. A lingering mystery was whether there might be many misaligned planets that Kepler would have a hard time finding. "With our study, we now know that there likely isn't a large population of misaligned planets that Kepler missed, since circumbinary disks around tight binary stars are also typically aligned with their stellar hosts," added Czekala.

Still, based on this finding, the astronomers conclude that misaligned planets around wide binary stars should be out there and that it would be an exciting population to search for with other exoplanet-finding methods like direct imaging and microlensing. (NASA's Kepler mission used the transit method, which is one of the ways to find a planet.)

Czekala now wants to find out why there is such a strong correlation between disk (mis)alignment and the binary star orbital period. "We want to use existing and coming facilities like ALMA and the next generation Very Large Array to study disk structures at exquisite levels of precision," he said, "and try to understand how warped or tilted disks affect the planet formation environment and how this might influence the population of planets that form within these disks."

"This research is a great example of how new discoveries build on previous observations," said Joe Pesce, National Science Foundation Program Officer for NRAO and ALMA. "Discerning trends in the circumbinary disk population was only made possible by building on the foundation of archival observational programs undertaken by the ALMA community in previous cycles."

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New type of pulsating star discovered

A star that pulsates on just one side has been discovered in the Milky Way about 1500 light years from Earth. It is the first of its kind to be found and scientists expect to find many more similar systems as technology to listen inside the beating hearts of stars improves.

"What first caught my attention was the fact it was a chemically peculiar star," said co-author Dr Simon Murphy from the Sydney Institute for Astronomy at the University of Sydney. "Stars like this are usually fairly rich with metals -- but this is metal poor, making it a rare type of hot star."

Dr Murphy shared the find with international collaborators to discover others had started to study the star, known as HD74423, which is about 1.7 times the mass of the Sun.

Together they have published their findings today in Nature Astronomy.

"We've known theoretically that stars like this should exist since the 1980s," said co-author Professor Don Kurtz from the University of Central Lancashire in Britain.

"I've been looking for a star like this for nearly 40 years and now we have finally found one," said Professor Kurtz, who is the inaugural Hunstead Distinguished Visitor at the University of Sydney.

Stars that pulsate have been known in astronomy for a long time. Our own Sun dances to its own rhythms. These rhythmic pulsations of the stellar surface occur in young and in old stars, and can have long or short periods, a wide range of strengths and different causes.

There is however one thing that all these stars had thus far in common: the oscillations were always visible on all sides of the star. Now an international team, including researchers from the University of Sydney, has discovered a star that oscillates largely over one hemisphere.

The scientists have identified the cause of the unusual single-sided pulsation: the star is located in a binary star system with a red dwarf. Its close companion distorts the oscillations with its gravitational pull. The clue that led to its discovery came from citizen scientists poring over public data from NASA's TESS satellite, which is hunting for planets around distant stars.

The orbital period of the binary system, at less than two days, is so short that the larger star is being distorted into a tear-drop shape by the gravitational pull of the companion.

Professor Gerald Handler from the Nicolaus Copernicus Astronomical Centre in Poland and lead author said: "The exquisite data from the TESS satellite meant that we could observe variations in brightness due to the gravitational distortion of the star as well as the pulsations."

To their surprise the team observed that the strength of the pulsations depended on the aspect angle under which the star was observed, and the corresponding orientation of the star within the binary. This means the pulsation strength varies with the same period as that of the binary.

"As the binary stars orbit each other we see different parts of the pulsating star," said Dr David Jones at the Instituto de Astrofisica de Canarias and co-author of the study. "Sometimes we see the side that points towards the companion star, and sometimes we see the outer face."

This is how the astronomers could be certain that the pulsations were only found on one side of the star, with the tiny fluctuations in brightness always appearing in their observations when the same hemisphere of the star was pointed towards the telescope.

The discovery of the unusual behaviour of the star was initially made by citizen scientists. These amateur astronomy sleuths painstakingly inspected the enormous amounts of data that TESS regularly supplies, as they search for new and interesting phenomena.

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