Hints of a volcanically active exo-moon

Jupiter's moon Io is the most volcanically active body in our solar system. Today, there are indications that an active moon outside our solar system, an exo-Io, could be hidden at the exoplanet system WASP-49b. "It would be a dangerous volcanic world with a molten surface of lava, a lunar version of close-in Super Earths like 55 Cancri-e" says Apurva Oza, postdoctoral fellow at the Physics Insitute of the University of Bern and associate of the NCCR PlanetS, "a place where Jedis go to die, perilously familiar to Anakin Skywalker." But the object that Oza and his colleagues describe in their work seems to be even more exotic than Star Wars science fiction: the possible exomoon would orbit a hot giant planet, which in turn would race once around its host star in less than three days -- a scenario 550 light years away in the inconspicuous constellation of Lepus, underneath the bright Orion constellation.

Sodium gas as circumstantial evidence

Astronomers have not yet discovered a rocky moon beyond our solar system and it's on the basis of circumstantial evidence that the researchers in Bern conclude that the exo-Io exists: Sodium gas was detected at the WASP 49-b at an anomalously high-altitude. "The neutral sodium gas is so far away from the planet that it is unlikely to be emitted solely by a planetary wind," says Oza. Observations of Jupiter and Io in our solar system, by the international team, along with mass loss calculations show that an exo-Io could be a very plausible source of sodium at WASP 49-b. "The sodium is right where it should be" says the astrophysicist.

Tides keep the system stable

Already in 2006, Bob Johnson of the University of Virginia and the late Patrick Huggins at New York University, USA had shown that large amounts of sodium at an exoplanet could point to a hidden moon or ring of material, and ten years ago, researchers at Virginia calculated that such a compact system of three bodies: star, close-in giant planet and moon, can be stable over billions of years. Apurva Oza was then a student at Virginia, and after his PhD on moons atmospheres in Paris, decided to pick up the theoretical calculations of these researchers. He now publishes the results of his work together with Johnson and colleagues in the Astrophysical Journal.

"The enormous tidal forces in such a system are the key to everything," explains the astrophysicist. The energy released by the tides to the planet and its moon keeps the moon's orbit stable, simultaneously heating it up and making it volcanically active. In their work, the researchers were able to show that a small rocky moon can eject more sodium and potassium into space through this extreme volcanism than a large gas planet, especially at high altitudes. "Sodium and potassium lines are quantum treasures to us astronomers because they are extremely bright," says Oza, "the vintage street lamps that light up our streets with yellow haze, is akin to the gas we are now detecting in the spectra of a dozen exoplanets."

"We need to find more clues"

The researchers compared their calculations with these observations and found five candidate systems where a hidden exomoon can survive against destructive thermal evaporation. For WASP 49-b the observed data can be best explained by the existence of an exo-Io. However, there are other options. For example, the exoplanet could be surrounded by a ring of ionized gas, or non-thermal processes. "We need to find more clues," Oza admits. The researchers are therefore relying on further observations with ground-based and space-based instruments.

"While the current wave of research is going towards habitability and biosignatures, our signature is a signature of destruction," says the astrophysicist. A few of these worlds could be destroyed in a few billion years due to the extreme mass loss. "The exciting part is that we can monitor these destructive processes in real time, like fireworks," says Oza.

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Exomoons may be home to extra-terrestrial life

Artist's concept of a moon orbiting a ringed planet.

Moons orbiting planets outside our solar system could offer another clue about the pool of worlds that may be home to extra-terrestrial life, according to an astrophysicist at the University of Lincoln.

Exoplanets are planets outside our solar system and up to this point nearly 4,000 have been discovered. Only a small proportion of these are likely to be able to sustain life, existing in what is known as the habitable zone. But some planets, especially large gas giants, may harbour moons which contain liquid water.

Dr Sutton said: "These moons can be internally heated by the gravitational pull of the planet they orbit, which can lead to them having liquid water well outside the normal narrow habitable zone for planets that we are currently trying to find Earth-like planets in. I believe that if we can find them, moons offer a more promising avenue to finding extra-terrestrial life."

This interest has inspired Dr Sutton's latest research, which looked at the possibility of moons orbiting the exoplanet J1407b, analysing whether they may have caused gaps in the planet's ring system.

Because of their size and distance from Earth, exomoons are very difficult to detect. Scientists have to locate them by looking for the effect they have on objects around them, such as planetary rings.

Dr Sutton ran computer simulations to model the rings around J1407b, which are 200 times larger than those around Saturn. Gravitational forces between all particles were calculated and used to update the positions, velocities and accelerations in the computer models of the planet and its ring system. He then added a moon that orbited at various ratios outside of the rings to test whether this caused gaps to form where expected over 100 orbital periods.

Findings revealed that while the orbiting moon did have an effect on the scattering of particles along the ring edge, the expected gaps in the ring structure were unlikely to be caused by the gravitational forces of a currently unseen moon orbiting outside the rings.

From Science Daily

Astronomers find first compelling evidence for a moon outside our solar system

This is an artist's impression of the exoplanet Kepler-1625b, transiting the star, with the candidate exomoon in tow.

A pair of Columbia University astronomers using NASA's Hubble Space Telescope and Kepler Space Telescope have assembled compelling evidence for the existence of a moon orbiting a gas-giant planet 8,000 light-years away.

In a paper published Oct. 3 in the journal Science Advances, Alex Teachey and David Kipping report that the detection of a candidate exomoon -- that is, moons orbiting planets in other star systems -- is unusual because of its large size, comparable to the diameter of Neptune. Such gargantuan moons do not exist in our own solar system, where nearly 200 natural satellites have been cataloged.

"This would be the first case of detecting a moon outside our solar system," said Kipping, an assistant professor of astronomy at Columbia. "If confirmed by follow-up Hubble observations, the finding could provide vital clues about the development of planetary systems and may cause experts to revisit theories of how moons form around planets."

In looking for exomoons, the researchers analyzed data from 284 Kepler-discovered planets that were in comparatively wide orbits, with periods greater than 30 days, around their host star. The observations measured the momentary dimming of starlight as a planet passed in front of its star, called a transit. The researchers found one instance, in Kepler 1625b, that had intriguing anomalies.

"We saw little deviations and wobbles in the light curve that caught our attention," Kipping said.

The Kepler results were enough for the team to get 40 hours of time with Hubble to intensively study the planet, obtaining data four times more precise than that of Kepler. The researchers monitored the planet before and during its 19-hour-long transit across the face of the star. After it ended, Hubble detected a second and much smaller decrease in the star's brightness 3.5 hours later, consistent with "a moon trailing the planet like a dog following its owner on a leash," Kipping said. "Unfortunately, the scheduled Hubble observations ended before the complete transit of the moon could be measured."

In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by measuring that the planet began its transit 1.25 hours earlier than predicted. This is consistent with the planet and moon orbiting a common center of gravity (barycenter) that would cause the planet to wobble from its predicted location.

"An extraterrestrial civilization watching the Earth and Moon transit the Sun would note similar anomalies in the timing of Earth's transit," Kipping said.

The researchers note that in principle this anomaly could be caused by the gravitational pull of a hypothetical second planet in the system, although Kepler found no evidence for additional planets around the star during its four-year mission.

"A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," said lead author Teachey, NSF Graduate Fellow in astronomy at Columbia. "It was a shocking moment to see that light curve, my heart started beating a little faster and I just kept looking at that signature. But we knew our job was to keep a level head testing every conceivable way in which the data could be tricking us until we were left with no other explanation."

The moon is estimated to be only 1.5 percent the mass of its companion planet, which itself estimated to be several times the mass of Jupiter. This value is close to the mass-ratio between the Earth and its moon. But in the case of the Earth-Moon system and the Pluto-Charon system -- the largest of the five known natural satellites of the dwarf planet Pluto -- an early collision with a larger body is hypothesized to have blasted off material that later coalesced into a moon. Kepler 1625b and its satellite, however, are gaseous, not rocky, and, therefore, such a collision may not lead to the condensation of a satellite.

Exomoons are difficult to find because they are smaller than their companion planet and so their transit signal is weak; they also shift position with each transit because the moon is orbiting the planet. In addition, the ideal candidate planets hosting moons are in large orbits, with long and infrequent transit times. In this search, the Neptune-sized moon would have been among the easiest to first detect because of its large size.

The host planet and its moon lie within the solar mass star's (Kepler 1625) habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface. "Both bodies, however, are considered to be gaseous and therefore unsuitable for life as we know it," Kipping said.

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TRAPPIST-1 Planets Have No Large Moons, Study Argues

Artist's impression of a hypothetical exomoon going around an imagined planet. Exomoons have not been discovered yet, but we know of thousands of exoplanet candidates.

While we know of thousands of exoplanets and exoplanet candidates, the search for moons outside of our solar system is just beginning. We don't have a confirmed exomoon discovery yet, but they're bound to be out there.

Finding exomoons will help us better understand habitability on Earth. Some experts say a reason that life arose is our own moon is so close to the size of our planet, which stabilized its axis rotation. However, other studies (such as this 2011 American Astronomical Society paper quoted in a NASA Astrobiology story) argue that the gravitational influence of other planets in our solar system provide enough stability.

A new study looks at the possibilities of large moons in TRAPPIST-1, a notoriously crowded exoplanet system that may have habitable planets within it. Earlier this year, observations from NASA's Spitzer Space Telescope indicated that seven planets here could be rocky and have liquid water on their surfaces, making TRAPPIST-1 the system with the most potentially habitable planets.

But even before NASA's discovery, TRAPPIST-1 was known and pondered by scientists, including the author of the new paper, Stephen Kane, an associate professor of astronomy at San Francisco State University who specializes in exoplanets.

"I have several publications now on exomoons, and for many years I've been thinking about how the ability of a planet to host a moon scales with the presence of nearby planets and proximity to the host star," Kane said in an e-mail. "The discovery of the TRAPPIST-1 system prompted me to finally calculate whether or not planets in compact planetary systems can actually harbor moons."

Artist's impression of the TRAPPIST-1 system, located about 40 light-years from Earth.

Kane cautioned that scientists can't overly attribute Earth's habitability to our moon, because Earth is the only known habitable planet. However, the moon does have an important role: It creates significant tides on Earth, which probably helped create the tidal pools in which early biochemistry could occur.

"The presence of the moon has helped to stabilize changes in the tilt of the Earth's rotational axis, which in turn creates longer periods of climate stability," Kane added. "So although it's difficult to say what the Earth would be like without a moon, we can certainly describe ways in which it has positively influenced our present environment."

For TRAPPIST-1, Kane found that the planets are so tightly packed together that large moons would likely be impossible. While the rotational axes of the planets would quickly change and have more chaotic climates, he said, life could still evolve — it just might take a longer time.

Kane's methodology involved studying the influences of two parameters: the Hill radius, or the area in space in which a planet exerts gravitational influence based on its mass and distance from the host star, and the Roche limit, which identifies where the gravitational effect near a planet is too strong for a moon to survive.

Read more at Discovery News

Life Outside Our Solar System Might Exist on Exomoons

As some scientists search for habitable planets outside our solar system, other researchers are tackling a similar question for the moons of these planets. So-called exomoons have yet to be found outside our solar system, and a detection could be a decade away - or more.

But scientists, writing in a new research paper, theorize a Mars-sized exomoon of a gas giant planet and ask whether or not liquid water could be found on its surface.

In our own solar system, the closest analog is Jupiter's Ganymede, the biggest moon in the solar system and about five-sixth the size of Mars.

NASA confirmed in 2015 the presence of a liquid ocean on Ganymede after performing Hubble Space Telescope observations of the moon's auroras, which appear to rock back and forth less than expected with Jupiter's magnetic field. The space agency said the attenuation is likely due to a salty ocean under Ganymede's surface.

As for this theoretical Mars-sized exomoon, the picture is murky. The scientists considered energy sources such as stellar radiation (which changes as a function of distance to the star), the stellar reflected light from a Jupiter-sized planet on the moon, the planet's own thermal emission on the moon, and the tidal heating inside the moon that is generated due to the changing gravitational pull of the planet. (This tidal heating would be most pronounced if the moon had an eccentric orbit, like the volcanic moon Io has around Jupiter.)

It's know that tidal heating rates decrease if a moon is molten inside, because lava creates an inherent negative feedback mechanism where the heating sort of switches off, and the moon cools down inside. This is called the "tidal thermostat effect," co-author of the paper Rene Heller said in an email. Heller is an astrophysicist at the Max Planck Institute for Solar System Research in Germany.

"We investigate, for the first time, the interplay of all the possible exomoon heat sources as a function of various distances from the host star," he added. "Actually, we even consider two possible types of host stars: a sun-like star, and a red dwarf star (an M dwarf)."

For a sun-like star, the authors found that any moon around a gas giant beyond three astronomical units, or three Earth-sun distances, would have a high enough energy flux to stop the tidal thermostat effect from happening. But if the moon is volatile enough, it could have global volcanism - just like what we see on Io.

Heller described this situation as "dangerous" for organisms.

"They might have lots of liquid surface water, but their surfaces could at the same time be blotched with devastating volcanoes," he wrote. "Nevertheless, we illustrate that they could be habitable given the right amount of tidal heating, and we show at which distances to their planets these moons would need to be."

M-dwarfs are a common target for exoplanet searches because they are smaller and dimmer, making it easier to see planets passing across their surfaces, or the effect of planets tugging on the star itself. But for exomoons, it's even less clear how habitable they would be in such a system. "Moons cannot be stable in the very inner regions of the stellar habitable zone," Heller said.

The best examples for tidally heated bodies in our own solar system are all moons: Jupiter's Io and Europa, as well as Saturn's Enceladus. While Europa and Enceladus are strongly suspected to have oceans underneath an icy surface, Heller pointed out his research is more focused on habitability on the surface of the moon. A better analog, he said, might be Saturn's moon Titan - but with a much warmer surface. Titan has a thick orange atmosphere, as well as liquid hydrocarbon lakes.

"Due to observational selection effects, which will prefer big moons around low-mass planets, I think the first exomoon will be unlike anything we know from the solar system," Heller said.

Read more at Discovery News

Kepler, don't give up on the hunt for exomoons

This simulation shows the collision of two celestial bodies, ejecting enough debris into orbit to form a moon large enough for the Kelper spacecraft to detect.

The Kepler spacecraft has been prolific in its search for planets outside our solar system, known as exoplanets, discovering thousands since its launch in 2009. But the hunt for moons orbiting these exoplanets, or exomoons, is vastly more challenging. While no exomoons have been found to date, a new study shows that the search is not futile.

Researchers have demonstrated for the first time that it is possible for a planetary collision to form a moon large enough for Kepler to detect. Lawrence Livermore National Laboratory physicist Megan Bruk Syal and Amy Barr of the Planetary Science Institute conducted a series of around 30 simulations to explore how various factors affect moon creation. In the end, they were able to narrow in on a set of conditions that would create satellites much larger than Earth's moon. The study -- "Formation of massive rocky exomoons by giant impact" -- will appear in the May issue of the Royal Astronomical Society's Monthly Notices.

"We weren't modeling something that's been observed," Syal said. "This problem was more abstract, more theoretical. It took a while, but once we were able to generate these massive moons, we were pretty excited."

The leading thinking on the creation of Earth's moon is that a planetoid the size of Mars collided with a smaller proto-Earth about 4.5 billion years ago, ejecting significant debris into orbit that consolidated into a disk and eventually the moon. The result was a satellite that is about 1.2 percent of Earth's mass. But in order for an exomoon to be large enough for Kepler to detect with existing transit techniques, it would need to be at least 10 percent the size of Earth, according to detection criteria from the "Hunt for Exomoons with Kepler" project.

Previous research on Earth's moon considered factors like the angle of impact and relative masses of colliding bodies. As the impact angle becomes more oblique, more material is injected into orbit. Similarly, as the two bodies approach equal size, the disk mass increases. But this study found that a third factor -- impact velocity -- also plays a crucial role in determining how large a moon an impact can create.

"Prior research has focused on a fairly narrow set of conditions, favorable to forming Earth's moon," Syal said. "This is the first study to consider a much wider array of impact scenarios, exploring the full range of what may be possible in other planetary systems. There is a lot of uncharted territory."

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