Cubesat offers template for future astronomy missions

The first NASA-funded small satellite for exoplanet science is continuing to gather data well beyond its expected lifetime. The Colorado Ultraviolet Transit Experiment, known as CUTE, a six-unit cubesat equipped with a telescope to funnel data to a spectrograph, traveled to sun-synchronous low-Earth orbit in September 2021 as a secondary payload on the NASA- U.S. Geological Survey Landsat 9 Earth-observation mission. CUTE was designed to operate in space for at least eight months. Twenty-seven months later, the satellite’s onboard instruments still are observing the dramatic atmospheric loss of “hot jupiters,” gas giants orbiting very close to bright stars. “This atmospheric escape is incredibly fast,” said Kevin France, CUTE principal investigator at the University of Colorado’s Laboratory for Atmospheric and Space Physics (LASP), which assembled, tested and operates the satellite. “The materials are coming out so fast that they are dragging all the heavy elements out of the atmosphere with them.” Based on CUTE’s success, two additional NASA-funded, LASP-led missions have adopted similar mission and instrument designs. “CUTE’s been a great success, particularly given that we didn’t really know if we could do it for the amount of money that we proposed,” France told SpaceNews at the American Geophysical Union conference here. Lessons learned from CUTE are helping researchers “figure out how to build small spacecraft, how to build small instruments and how to have a student-led team,” France said.

The Colorado Ultraviolet Transit Experiment, known as CUTE, is a cubesat launched in 2021 to characterize the composition and mass-loss rates of exoplanet atmospheres. Credit: University of Colorado Laboratory for Atmospheric and Space Physics

Smallsat Astronomy

The budget for developing, assembling and operating CUTE through the summer of 2024 is about $5.5 million.

“At this cost, we’re still figuring out how to make things work,” France said. “So, working and doing science is batting above your average.”

The missions mimicking CUTE are 12-unit cubesats Sprite and Mantis.

Sprite, which stands for Supernova remnants Proxies for Reionization and Integrated Testbed Experiment, is scheduled for launch in 2024. The $4 million mission will study how gas and dust is processed in galaxies and how energetic ionizing radiation is transported from stars to the intergalactic medium between galaxies.

Mantis, short for Monitoring Activity from Nearby sTars with uv Imaging and Spectroscopy, is an $8.5 million campaign to observe how high-energy radiation from stars influences the habitability of planets.

Ingenuity and Chance

CUTE remains operational more than two years after launch thanks to ingenuity and luck.

The tiny satellite was sent into orbit at a higher altitude than mission planners expected. As a result, CUTE is expected to reenter Earth’s atmosphere in 2027, instead of late this year as originally scheduled.

Additional time in orbit means extra wear and tear on hardware.

“Every time we have a problem, we figure out a new way to operate the spacecraft,” France said.

When the satellite’s primary and backup memory storage cards failed, for example, mission operators learned to communicate directly with CUTE’s scientific payload.

“We send the data down directly from the science payload to the ground and we bypass the spacecraft altogether,” France said.

Distant Galaxies

Cubesats have been widely adopted for civil and commercial space missions since LASP researchers proposed CUTE in 2016.

At the time, “we were beginning to believe we could study the sun and Earth’s upper atmosphere with cubesats,” France said. “But the idea that we could be pointed at targets that are 300, 400 light years away and do high-precision astronomical measurements from the cubesat was ambitious.”

Now that CUTE has shown the potential, small satellites could play key roles in observing distant galaxies, black holes “and all the other things that we’re interested in studying,” France said.

Exoplanets: How we'll search for signs of life

Whether there is life elsewhere in the universe is a question people have pondered for millennia; and within the last few decades, great strides have been made in our search for signs of life outside of our solar system. NASA missions like the space telescope Kepler have helped us document thousands of exoplanets - planets that orbit around other stars. And current NASA missions like Transiting Exoplanet Survey Satellite (TESS) are expected to vastly increase the current number of known exoplanets. It is expected that dozens will be Earth-sized rocky planets orbiting in their stars' habitable zones, at distances where water could exist as a liquid on their surfaces. These are promising places to look for life. This will be accomplished by missions like the soon-to-be-launched James Webb Space Telescope, which will complement and extend the discoveries of the Hubble Space Telescope by observing at infrared wavelengths. It is expected to launch in 2021, and will allow scientists to determine if rocky exoplanets have oxygen in their atmospheres. Oxygen in Earth's atmosphere is due to photosynthesis by microbes and plants. To the extent that exoplanets resemble Earth, oxygen in their atmospheres may also be a sign of life. Not all exoplanets will be Earth-like, though. Some will be, but others will differ from Earth enough that oxygen doesn't necessarily come from life. So with all of these current and future exoplanets to study, how do scientists narrow down the field to those for which oxygen is most indicative of life?

To answer this question, an interdisciplinary team of researchers, led by Arizona State University (ASU), has provided a framework, called a "detectability index" which may help prioritize exoplanets that require additional study. The details of this index have recently been published in the Astrophysical Journal of the American Astronomical Society.

"The goal of the index is to provide scientists with a tool to select the very best targets for observation and to maximize the chances of detecting life," says lead author Donald Glaser of ASU's School of Molecular Sciences.

The oxygen detectability index for a planet like Earth is high, meaning that oxygen in Earth's atmosphere is definitely due to life and nothing else. Seeing oxygen means life. A surprising finding by the team is that the detectability index plummets for exoplanets not-too-different from Earth.

Although Earth's surface is largely covered in water, Earth's oceans are only a small percentage (0.025%) of Earth's mass. By comparison, moons in the outer solar system are typically close to 50% water ice.

"It's easy to imagine that in another solar system like ours, an Earth-like planet could be just 0.2% water," says co-author Steven Desch of ASU's School of Earth and Space Exploration. "And that would be enough to change the detectability index. Oxygen would not be indicative of life on such planets, even if it were observed. That's because an Earth-like planet that was 0.2% water--about eight times what Earth has--would have no exposed continents or land."

Without land, rain would not weather rock and release important nutrients like phosphorus. Photosynthetic life could not produce oxygen at rates comparable to other non-biological sources.

"The detectability index tells us it's not enough to observe oxygen in an exoplanet's atmosphere. We must also observe oceans and land," says Desch. "That changes how we approach the search for life on exoplanets. It helps us interpret observations we've made of exoplanets. It helps us pick the best target exoplanets to look for life on. And it helps us design the next generation of space telescopes so that we get all the information we need to make a positive identification of life."

Scientists from diverse fields were brought together to create this index. The formation of the team was facilitated by NASA's Nexus for Exoplanetary System Science (NExSS) program, which funds interdisciplinary research to develop strategies for looking for life on exoplanets. Their disciplines include theoretical and observational astrophysics, geophysics, geochemistry, astrobiology, oceanography, and ecology.

"This kind of research needs diverse teams, we can't do it as individual scientists" says co-author Hilairy Hartnett who holds joint appointments at ASU's School of Earth and Space Exploration and School of Molecular Sciences.

In addition to lead author Glaser and co-authors Harnett and Desch, the team includes co-authors Cayman Unterborn, Ariel Anbar, Steffen Buessecker, Theresa Fisher, Steven Glaser, Susanne Neuer, Camerian Millsaps, Joseph O'Rourke, Sara Imari Walker, and Mikhail Zolotov who collectively represent ASU's School of Molecular Sciences, School of Earth and Space Exploration, and School of Life Sciences. Additional scientists on the team include researchers from the University of California Riverside, Johns Hopkins University and the University of Porto (Portugal).

It is the hope of this team that this detectability index framework will be employed in the search for life. "The detection of life on a planet outside our solar system would change our entire understanding of our place in the universe," says Glaser. "NASA is deeply invested in searching for life, and it is our hope that this work will be used to maximize the chance of detecting life when we look for it."

Breathable atmospheres may be more common in the universe than we first thought

The existence of habitable alien worlds has been a mainstay of popular culture for more than a century. In the 19th century, astronomers believed that Martians might be using canal-based transport links to traverse the red planet. Now, despite living in an age when scientists can study planets light years from our own solar system, most new research continues to diminish the chances of finding other worlds on which humans could live. The biggest stumbling block may be oxygen - human settlers would need a high oxygen atmosphere in which to breathe. So how were we so lucky to evolve on a planet with plenty of oxygen? The history of Earth's oceans and atmosphere suggests that the rise to present-day levels of O2 was pretty difficult. The current consensus is that Earth underwent a three-step rise in atmospheric and oceanic oxygen levels, the first being called the "Great Oxidation Event" at around 2.4 billion years ago. After that came the "Neoproterozoic Oxygenation Event" around 800 million years ago, and then finally the "Paleozoic Oxygenation Event" about 400 million years ago, when oxygen levels on Earth reached their modern peak of 21%. What happened during these three periods to increase oxygen levels is a matter for debate. One idea is that new organisms "bioengineered" the planet, restructuring the atmosphere and oceans through either their metabolisms or their lifestyles.

For example, the rise of land plants roughly 400 million years ago could have increased oxygen in the atmosphere through land-based photosynthesis, taking over from photosynthetic bacteria in the ocean which have been the main oxygen producers for most of Earth's history. Alternatively, plate tectonic changes or gigantic volcanic eruptions have also been linked to the Earth's oxygenation events.

This event-based history of how oxygen came to be so plentiful on Earth implies that we're very fortunate to be living on a high-oxygen world. If one volcanic eruption hadn't happened, or a certain type of organism hadn't evolved, then oxygen might have stalled at low levels.

But our latest research suggests that this isn't the case. We created a computer model of the Earth's carbon, oxygen and phosphorus cycles and found that the oxygen transitions can be explained by the inherent dynamics of our planet and likely didn't require any miraculous events.

Phosphorus - University of Leeds
One thing we think is missing from theories about Earth's oxygenation is phosphorus. This nutrient is very important for photosynthetic bacteria and algae in the ocean. How much marine phosphorous there is will ultimately control how much oxygen is produced on Earth. This is still true today - and has been so since the evolution of photosynthetic microbes some three billion years ago.

Photosynthesis in the ocean depends on phosphorus, but high phosphate levels also drive consumption of oxygen in the deep ocean through a process called eutrophication. When photosynthetic microbes die, they decompose, which consumes oxygen from the water. As oxygen levels fall, sediments tend to release even more phosphorus. This feedback loop rapidly removes oxygen. This meant that oxygen levels in the oceans were able to change rapidly, but they were buffered over long timescales by another process involving the Earth's mantle.

Throughout Earth's history, volcanic activity has released gases that react with and remove oxygen from the atmosphere. These gas fluxes have subsided over time due to Earth's mantle cooling, and our computer model suggests this slow reduction along with the initial evolution of photosynthetic life was all that was necessary to produce a series of step-change increases in oxygen levels.

These stepped increases bear a clear resemblance to the three-step rise in oxygen that has occurred throughout Earth's history. The model also supports our current understanding of ocean oxygenation, which appears to have involved numerous cycles of oxygenation and deoxygenation before the oceans became resiliently oxygenated as they are today.

What is really exciting about all of this is that the oxygenation pattern can be created without the need for difficult and complex evolutionary leaps forward, or circumstantial catastrophic volcanic or tectonic events. So it appears that Earth's oxygenation may have been inescapable once photosynthesis had evolved - and the chances of high oxygen worlds existing elsewhere could be much higher.

Exoplanet axis study boosts hopes of complex life, just not next door

"They're out there," goes a saying about extraterrestrials. It would seem more likely to be true in light of a new study on planetary axis tilts. Astrophysicists at the Georgia Institute of Technology modeled a theoretical twin of Earth into other star systems called binary systems because they have two stars. They concluded that 87% of exo-Earths one might find in binary systems should have axis tilts similarly steady to Earth's, an important ingredient for climate stability that favors the evolution of complex life. "Multiple-star systems are common, and about 50% of stars have binary companion stars. So, this study can be applied to a large number of solar systems," said Gongjie Li, the study's co-investigator an assistant professor at Georgia Tech's School of Physics. Single-star solar systems like our own with multiple planets appear to be rarer. The researchers started out contrasting how the Earth's axis tilt, also called obliquity, varies over time with the variation of Mars' axis tilt. Whereas our planet's mild obliquity variations have been great for a livable climate and for evolution, the wild variations of Mars' axis tilt may have helped wreck its atmosphere, as explained in the section below. Then the researchers modeled Earth into habitable, or Goldilocks, zones in Alpha Centauri AB - our solar system's nearest neighbor, a binary system with one star called "A" and the other "B." After that, they expanded the model to a more universal scope.

"We simulated what it would be like around other binaries with multiple variations of the stars' masses, orbital qualities, and so on," said Billy Quarles, the study's principal investigator and a research scientist in Li's lab. "The overall message was positive but not for our nearest neighbor."

Alpha Centauri A actually didn't look bad, but the outlook for mild axis dynamics on an exo-Earth modeled around star B was wretched. This may douse some hopes because Alpha Centauri AB is four lightyears away, and a mission named Starshot with big-name backers plans to launch a space probe to look for signs of advanced life there.

The researchers are publishing their study, which was co-led by Jack Lissauer from NASA Ames Research Center, in Astrophysical Journal on November 19, 2019, under the title: "Obliquity Evolution of Circumstellar Planets in Sun-like Stellar Binaries." The research was funded by the NASA Exobiology Program.

No exoplanets have been confirmed around A or B; an exoplanet has been confirmed around the nearby red dwarf star Proxima Centauri, but it is very likely to be uninhabitable.

Earth? Just right
Even with its ice ages and hot phases, Earth's climatological framework has been calm for hundreds of millions of years - in part because of its mild orbital and axis-tilt dynamics - allowing evolution to take big strides. Wildly varying dynamics, and thus climate, like on Mars would stand to regularly kill off advanced life, stunting evolution.

Earth's orbit around the sun is on a slight incline that seesaws gently and very slowly through a slight precession, a kind of oscillation. As Earth revolves, it shifts position relative to the sun, circling it a little like a spirograph drawing. The orbit also precesses in shape between slightly more and slightly less oblong over 100,000-year periods.

Earth's axis tilt precesses between 22.1 and 24.5 degrees over the course of 41,000 years. Our large moon stabilizes our tilt through its gravitational relationship with Earth, otherwise, bouncy gravitational interconnections with Mercury, Venus, Mars, and Jupiter would jolt our tilt with resonances.

"If we didn't have the moon, Earth's tilt could vary by about 60 degrees," Quarles said. "We'd look maybe like Mars, and the precession of its axis appears to have helped deplete its atmosphere."

Mars' axis precesses between 10 degrees and 60 degrees every 2 million years. At the 10-degree tilt, the atmosphere condenses at the poles, creating caps that lock up a lot of the atmosphere in ice. At 60 degrees, Mars could grow an ice belt around its equator.

Universe? Hopeful
In Alpha Centauri AB, star B, about the size of our sun, and the larger star, A, orbit one another at about the distance between Uranus and our sun, which is a very close for two stars in a binary system. The study modeled variations of an exo-Earth orbiting either star but concentrated on a modeled Earth orbit in the habitable zone centered around B, with A being the orbiting star.

A's orbit is very elliptical, passing close by and then moving very far away from B and slinging powerful gravity, which, in the model, overpowered exo-Earth's own dynamics. Its tilt and orbit varied widely; adding our moon to the model didn't help.

"Around Alpha Centauri B, if you don't have a moon, you have a more stable axis than if you do have a moon. If you have a moon, it's pretty much bad news," Quarles said.

Even without a moon and with mild axis variability, complex, Earthlike evolution would seem to have a hard time on the modeled exo-Earth around B.

"The biggest effect you would see is differences in the climate cycles related to how elongated the orbit is. Instead of having ice ages every 100,000 years like on Earth, they may come every 1 million years, be worse, and last much longer," Quarles said.

But a sliver of hope for Earthlike conditions turned up in the model: "Planetary orbit and spin need to precess just right relative to the binary orbit. There is this tiny sweet spot," Quarles said.

When the researchers expanded the model to binary systems in the universe, the probability of gentle obliquity variations ballooned.

"In general, the separation between the stars is larger in binary systems, and then the second star has less of an effect on the model of Earth. The planet's own motion dynamics dominate other influences, and obliquity usually has a smaller variation," Li said. "So, this is quite optimistic."

New method for exoplanet stability analysis

Exoplanets revolving around distant stars are coming quickly into focus with advanced technology like the Kepler space telescope. Gaining a full understanding of those systems is difficult, because the initial positions and velocities of the exoplanets are unknown. Determining whether the system dynamics are quasi-periodic or chaotic is cumbersome, expensive and computationally demanding. In this week's Chaos, from AIP Publishing, Tamas Kovacs delivers an alternative method for stability analysis of exoplanetary bodies using only the observed time series data to deduce dynamical measurements and quantify the unpredictability of exoplanet systems. "If we don't know the governing equations of the motion of a system, and we only have the time series - what we measure with the telescope - then we want to transform that time series into a complex network. In this case, it is called a recurrence network," Kovacs said. "This network holds all of the dynamical features of the underlying system we want to analyze." The paper draws on the work of physicist Floris Takens, who proposed in 1981 that the dynamics of a system could be reconstructed using a series of observations about the state of the system.With Takens's embedding theorem as a starting point, Kovacs uses time delay embedding to reconstruct a high-dimensional trajectory and then identify recurrence points, where bodies in the phase space are close to each other.

"Those special points will be the vertices and the edges of the complex network," Kovacs said. "Once you have the network, you can reprogram this network to be able to apply measures like transitivity, average path length or others unique to that network."

Kovacs tests the reliability of the method using a known system as a model, the three-body system of Saturn, Jupiter and the Sun, and then applies it to the Kepler 36b and 36c system. His Kepler system results agree with what is known.

"Earlier studies pointed out that Kepler 36b and 36c is a very special system, because from the direct simulation and the numerical integrations, we see the system is at the edge of the chaos," Kovacs said. "Sometimes, it shows regular dynamics, and at other times, it seems to be chaotic."

The author plans to next apply his methods to systems with more than three bodies, testing its scalability and exploring its ability to handle longer time series and sharper datasets.

Some planetary systems just aren't into heavy metal

Small planetary systems with multiple planets are not fans of heavy metal - think iron, not Iron Maiden - according to a new Yale University study. Researchers at Yale and the Flatiron Institute have discovered that compact, multiple-planet systems are more likely to form around stars that have lower amounts of heavy elements than our own Sun. This runs counter to a good deal of current research, which has focused on stars with higher metallicity. The research team looked at 700 stars and their surrounding planets for the study, which appears in The Astrophysical Journal Letters. The researchers considered any element heavier than helium - including iron, silicon, magnesium, and carbon - as a heavy metal. "We used iron as a proxy," said lead author John Michael Brewer, a postdoctoral researcher at Yale who works with astronomy professor Debra Fischer. "These are all elements that make up the rocks in small, rocky planets." Brewer said an abundance of compact, multi-planet systems around low-metallicity stars suggests several things. First, he said, it may indicate that there are many more of these systems than previously assumed. Until recently, research instruments have not had the necessary precision to detect smaller planets and instead focused on detecting larger planets. Now, with the advent of technology such as the Extreme Precision Spectrometer (EXPRES) developed by Fischer's team at Yale, researchers will be able to find smaller planets.

In addition, Brewer said, the new study suggests that small planetary systems may be the earliest type of planetary system, making them an ideal place to search for life on other planets. "Low-metallicity stars have been around a lot longer," Brewer said. "That's where we'll find the first planets that formed."

Fischer, who is a co-author of the study, demonstrated in 2005 that higher metallicity in stars increased the probability of forming large, Jupiter-like planets. This provided strong support to the core-accretion model for gas giant planet formation and established this as the leading mechanism for planet formation.

Understanding the formation of smaller planets has been more elusive.

"Our surprising result, that compact systems of multiple, small planets are more likely around lower metallicity stars suggests a new, important clue in understanding the most common type of planetary system in our galaxy," said co-author Songhu Wang, a 51 Pegasi b Fellow at Yale.

Another tantalizing possibility to explore, according to the researchers, is the connection between iron and silicon in the birth of planets. The new study shows a high silicon-to-iron ratio in stars with lower metallicity.

"Silicon could be the secret ingredient," Fischer said. "The ratio of silicon to iron is acting as a thermostat for planet formation. As the ratio increases, nature is dialing up the formation of small, rocky planets."

Construction of Europe's exoplanet hunter Plato begins

The construction of ESA's Plato mission to find and study planets beyond our Solar System will be led by Germany's OHB System AG as prime contractor, marking the start of the full industrial phase of the project. The announcement was made this week at the 69th International Astronautical Congress in Bremen, Germany, where the contract was formally signed. The contract covers the delivery of the satellite, including the testing phase leading to launch, support during the launch campaign, and the in-orbit commissioning phase. Plato, the PLAnetary Transits and Oscillations of stars mission, will be launched in 2026 to find and study extrasolar planetary systems, with a special emphasis on rocky planets around Sun-like stars and their habitable zone - the distance from a star where liquid water can exist on a planet's surface. "Does a second Earth exist in the Universe? is one of the exciting questions in astrophysics today," says Johann-Dietrich Worner, Director General of ESA. "With our Plato satellite we are focusing on Earth-like planets orbiting up to the habitable zone around other stars which are similar to our Sun. This will be a major step towards finding another Earth." The spacecraft will be built and assembled by OHB together with Thales Alenia Space (France and the UK) and RUAG Space Switzerland; many ESA member States will also be involved in the construction of this European planet hunter.

The German Aerospace Center (DLR) and a consortium of various European research centers and institutes will provide the scientific instrument, consisting of an array of 26 cameras and electronic units, that will observe a large patch of the sky on the lookout for planets.

"Plato is a next-generation exoplanet mission that will monitor thousands of bright stars over a large area of the sky in search of tiny, regular dips in their brightness caused by transiting planets," says Ana Heras, Plato project scientist at ESA.

"Since planets only block a minute portion of the light radiated by their parent star, this quest requires extremely precise, long-term photometric observations."

Plato will not only seek new planets but will also investigate the properties of their host stars, and determine the planetary masses, sizes and ages with unprecedented accuracy. This will help scientists understand the architecture of exoplanet systems and determine whether they might host habitable worlds. In addition, Plato will also perform asteroseismology - the study of seismic activity of stars - providing insight into stellar interiors and evolution.

The mission will expand on the work of Cheops, ESA's upcoming exoplanet watcher, which will be launched next year to perform a first characterisation of known planets. It will be followed by Ariel, scheduled for launch in 2028, which will observe a large and diverse sample of exoplanets to study their atmospheres in great detail.

Plato will operate from the 'L2' virtual point in space 1.5 million km beyond Earth as seen from the Sun. From this vantage point, it will be our outpost to unravel the mysteries of a multitude of extrasolar worlds.

"We are pleased to kick off construction of this exciting mission," says Filippo Marliani, ESA's Plato Project Manager.

"With the prime contractor and the support of European space industry, we are looking forward to building a spacecraft that will tackle some of humankind's most profound questions."

Astronomers find first evidence of possible moon outside our Solar System

Using NASA's Hubble and Kepler space telescopes, astronomers have uncovered tantalizing evidence of what could be the first discovery of a moon orbiting a planet outside our solar system. This moon candidate, which is 8,000 light-years from Earth in the Cygnus constellation, orbits a gas-giant planet that, in turn, orbits a star called Kepler-1625. Researchers caution that the moon hypothesis is tentative and must be confirmed by follow-up Hubble observations. "This intriguing finding shows how NASA's missions work together to uncover incredible mysteries in our cosmos," said Thomas Zurbuchen, associate administrator of NASA's Science Mission Directorate at Headquarters, Washington. "If confirmed, this finding could completely shake up our understanding of how moons are formed and what they can be made of." Since moons outside our solar system - known as exomoons - cannot be imaged directly, their presence is inferred when they pass in front of a star, momentarily dimming its light. Such an event is called a transit, and has been used to detect many of the exoplanets cataloged to date. However, exomoons are harder to detect than exoplanets because they are smaller than their companion planet, and so their transit signal is weaker when plotted on a light curve that measures the duration of the planet crossing and the amount of momentary dimming. Exomoons also shift position with each transit because the moon is orbiting the planet.

In search of exomoons, Alex Teachey and David Kipping, astronomers at Columbia University in New York, analyzed data from 284 Kepler-discovered planets that were in comparatively wide orbits, longer than 30 days, around their host star. The researchers found one instance in planet Kepler-1625b, of a transit signature with intriguing anomalies, suggesting the presence of a moon.

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

Based upon their findings, the team spent 40 hours making observations with Hubble to study the planet intensively - also using the transit method - obtaining more precise data on the dips of light. Scientists monitored the planet before and during its 19-hour transit across the face of the star. After the transit ended, Hubble detected a second, and much smaller, decrease in the star's brightness approximately 3.5 hours later. This small decrease is consistent with a gravitationally-bound moon trailing the planet, much like a dog following after its owner. Unfortunately, the scheduled Hubble observations ended before the complete transit of the candidate moon could be measured and its existence confirmed.

In addition to this dip in light, Hubble provided supporting evidence for the moon hypothesis by finding the planet transit occurring more than an hour earlier than predicted. This is consistent with a planet and moon orbiting a common center of gravity that would cause the planet to wobble from its predicted location, much the way Earth wobbles as our Moon orbits it.

The researchers note the planetary wobble could be caused by the gravitational pull of a hypothetical second planet in the system, rather than a moon. While Kepler has not detected a second planet in the system, it could be that the planet is there, but not detectable using Kepler's techniques.

"A companion moon is the simplest and most natural explanation for the second dip in the light curve and the orbit-timing deviation," Kipping explained. "It was definitely a shocking moment to see that Hubble light curve, my heart started beating a little faster as I kept looking at that signature. But we knew our job was to keep a level head and essentially assume it was bogus, testing every conceivable way in which the data could be tricking us."

In a paper published in the journal Science Advances, the scientists report the candidate moon is unusually large - potentially comparable to Neptune. Such large moons do not exist in our own solar system. The researchers say this may yield new insights into the development of planetary systems and may cause experts to revisit theories of how moons form around planets.

The moon candidate is estimated to be only 1.5 percent the mass of its companion planet, and the planet is estimated to be several times the mass of Jupiter. This mass-ratio is similar to the one between Earth and the Moon. In the case of the Earth-Moon system and the Pluto-Charon system, the moons are thought to be created through dust leftover after rocky planetary collisions. However, Kepler-1625b and its possible satellite are gaseous and not rocky, so the moon may have formed through a different process.

Researchers note that if this is indeed a moon, both it and its host planet lie within their star's habitable zone, where moderate temperatures allow for the existence of liquid water on any solid planetary surface. However, both bodies are considered to be gaseous and, therefore, unsuitable for life as we know it.

Future searches for exomoons, in general, will target Jupiter-size planets that are farther from their star than Earth is from the Sun. The ideal candidate planets hosting moons are in wide orbits, with long and infrequent transit times. In this search, a moon would have been among the easiest to detect because of its large size. Currently, there are just a handful of such planets in the Kepler database. Whether future observations confirm the existence of the Kepler-1625b moon, NASA's James Webb Space Telescope will be used to find candidate moons around other planets, with much greater detail than Kepler.

"We can expect to see really tiny moons with Webb," Teachey said.

Iron and Titanium in the Atmosphere of an Exoplanet

Exoplanets, planets in other solar systems, can orbit very close to their host star. When, in addition to this, the host star is much hotter than our Sun, then the exoplanet becomes as hot as a star. The hottest "ultra-hot" planet was discovered last year by American astronomers. Today, an international team, led by researchers from the University of Geneva (UNIGE), who joined forces with theoreticians from the University of Bern (UNIBE), Switzerland, discovered the presence of iron and titanium vapours in the atmosphere of this planet. The detection of these heavy metals was made possible by the surface temperature of this planet, which reaches more than 4,000 degrees [Celsius]. This discovery is published in the journal Nature [www.nature.com]. KELT-9 is a star located 650 light-years from Earth in the constellation Cygnus (the Swan). With a temperature of over 10,000 degrees [Celsius], it is almost twice as hot as the Sun. This star is orbited by a giant gas planet, KELT-9b, which is 30 times closer than the Earth's distance from the Sun. Because of this proximity, the planet circles its star in 36 hours and is heated to a temperature of over 4,000 degrees. It's not as hot as the Sun, but hotter than many stars. At present, we do not yet know what an atmosphere looks like and how it can evolve under such conditions.

That is why NCCR PlanetS researchers affiliated with the University of Bern recently performed a theoretical study on the atmosphere of the planet KELT-9b.

"The results of these simulations show that most of the molecules found there should be in atomic form, because the bonds that hold them together are broken by collisions between particles that occur at these extremely high temperatures," explains Kevin Heng, professor at the UNIBE. This is a direct consequence of the extreme temperature. Their study also predicts that it should be possible to observe gaseous atomic iron, in the planet's atmosphere using current telescopes.

Light Reveals the Chemical Components of the Atmosphere
The UNIGE FOUR ACES team, which is also part of the NCCR PlanetS at the Department of Astronomy of the Faculty of Science of the UNIGE, had observed this planet precisely as it was moving in front of its host star (i.e., during a transit). During transit, a tiny fraction of the light from the star filters through the planet's atmosphere, and analysis of this filtered light can reveal the chemical composition of the atmosphere.

This is achieved with a spectrograph, an instrument that spreads white light into its component colours, called a spectrum. If present among the components of the atmosphere, iron vapour would leave a well-recognisable fingerprint in the spectrum of the planet.

Using the HARPS-North spectrograph, built in Geneva and installed on the Telescopio Nazionale Galileo in La Palma, astronomers discovered a strong signal corresponding to iron vapour in the planet's spectrum. "With the theoretical predictions in hand, it was like following a treasure map," says Jens Hoeijmakers, a researcher at the Universities of Geneva and Bern and lead author of the study, "and when we dug deeper into the data, we found even more," he adds with a smile. Indeed, the team also detected the signature of another metal in vapour form: titanium.

This discovery reveals the atmospheric properties of a new class of so-called "ultra-hot Jupiter." However, scientists believe that many exoplanets have completely evaporated in environments similar to KELT-9b. Although this planet is probably massive enough to withstand total evaporation, this new study demonstrates the strong impact of stellar radiation on the composition of the atmosphere. Indeed, these observations confirm that the high temperatures reigning on this planet break apart most molecules, including those containing iron or titanium.

In cooler giant exoplanets, these atomic species are thought to be hidden within gaseous oxides or in the form of dust particles, making them hard to detect. This is not the case on KELT-9b. "This planet is a unique laboratory to analyze how atmospheres can evolve under intense stellar radiation," concludes David Ehrenreich, principal investigator with the UNIGE's FOUR ACES team.

Exoplanets where life could develop as on Earth

Scientists have identified a group of planets outside our solar system where the same chemical conditions that may have led to life on Earth exist. The researchers, from the University of Cambridge and the Medical Research Council Laboratory of Molecular Biology (MRC LMB), found that the chances for life to develop on the surface of a rocky planet like Earth are connected to the type and strength of light given off by its host star. Their study, published in the journal Science Advances, proposes that stars which give off sufficient ultraviolet (UV) light could kick-start life on their orbiting planets in the same way it likely developed on Earth, where the UV light powers a series of chemical reactions that produce the building blocks of life. The researchers have identified a range of planets where the UV light from their host star is sufficient to allow these chemical reactions to take place, and that lie within the habitable range where liquid water can exist on the planet's surface. "This work allows us to narrow down the best places to search for life," said Dr. Paul Rimmer, a postdoctoral researcher with a joint affiliation at Cambridge's Cavendish Laboratory and the MRC LMB, and the paper's first author. "It brings us just a little bit closer to addressing the question of whether we are alone in the universe."

The new paper is the result of an ongoing collaboration between the Cavendish Laboratory and the MRC LMB, bringing together organic chemistry and exoplanet research. It builds on the work of Professor John Sutherland, a co-author on the current paper, who studies the chemical origin of life on Earth.

In a paper published in 2015, Professor Sutherland's group at the MRC LMB proposed that cyanide, although a deadly poison, was in fact a key ingredient in the primordial soup from which all life on Earth originated.

In this hypothesis, carbon from meteorites that slammed into the young Earth interacted with nitrogen in the atmosphere to form hydrogen cyanide. The hydrogen cyanide rained to the surface, where it interacted with other elements in various ways, powered by the UV light from the Sun. The chemicals produced from these interactions generated the building blocks of RNA, the close relative of DNA which most biologists believe was the first molecule of life to carry information.

In the laboratory, Sutherland's group recreated these chemical reactions under UV lamps, and generated the precursors to lipids, amino acids and nucleotides, all of which are essential components of living cells.

"I came across these earlier experiments, and as an astronomer, my first question is always what kind of light are you using, which as chemists they hadn't really thought about," said Rimmer. "I started out measuring the number of photons emitted by their lamps, and then realised that comparing this light to the light of different stars was a straightforward next step."

The two groups performed a series of laboratory experiments to measure how quickly the building blocks of life can be formed from hydrogen cyanide and hydrogen sulphite ions in water when exposed to UV light. They then performed the same experiment in the absence of light.

"There is chemistry that happens in the dark: it's slower than the chemistry that happens in the light, but it's there," said senior author Professor Didier Queloz, also from the Cavendish Laboratory. "We wanted to see how much light it would take for the light chemistry to win out over the dark chemistry."

The same experiment run in the dark with the hydrogen cyanide and the hydrogen sulphite resulted in an inert compound which could not be used to form the building blocks of life, while the experiment performed under the lights did result in the necessary building blocks.

The researchers then compared the light chemistry to the dark chemistry against the UV light of different stars. They plotted the amount of UV light available to planets in orbit around these stars to determine where the chemistry could be activated.

They found that stars around the same temperature as our Sun emitted enough light for the building blocks of life to have formed on the surfaces of their planets. Cool stars, on the other hand, do not produce enough light for these building blocks to be formed, except if they have frequent powerful solar flares to jolt the chemistry forward step by step. Planets that both receive enough light to activate the chemistry and could have liquid water on their surfaces reside in what the researchers have called the abiogenesis zone.

Among the known exoplanets which reside in the abiogenesis zone are several planets detected by the Kepler telescope, including Kepler 452b, a planet that has been nicknamed Earth's 'cousin,' although it is too far away to probe with current technology. Next-generation telescopes, such as NASA's TESS and James Webb telescopes, will hopefully be able to identify and potentially characterise many more planets that lie within the abiogenesis zone.

Of course, it is also possible that if there is life on other planets, that it has or will develop in a totally different way than it did on Earth.

"I'm not sure how contingent life is, but given that we only have one example so far, it makes sense to look for places that are most like us," said Rimmer. "There's an important distinction between what is necessary and what is sufficient. The building blocks are necessary, but they may not be sufficient: it's possible you could mix them for billions of years and nothing happens. But you want to at least look at the places where the necessary things exist."

According to recent estimates, there are as many as 700 million trillion terrestrial planets in the observable universe. "Getting some idea of what fraction have been, or might be, primed for life fascinates me," said Sutherland. "Of course, being primed for life is not everything and we still don't know how likely the origin of life is, even given favourable circumstances - if it's really unlikely then we might be alone, but if not, we may have company."