Are there limits on how big a moon can be to orbit a given planet? All we have to work with, in the absence of confirmed exomoons, are the satellites of our Solar System’s planets, and here we see what appears to be a correlation between a planet’s mass and the mass of its moons. At least up to a point – we’ll get to that point in a moment. But consider: As Vera Dobos (University of Groningen, Netherlands) and colleagues point out in a recent paper for Monthly Notices of the Royal Astronomical Society, if we’re talking about moons forming in the circumplanetary disk around the young Sun, the total mass is on the order of 10-4Mp. Here Mp is the mass of the planet. A planet with 10 times Jupiter’s mass, given this figure, could have a moon as large as a third of Earth’s mass, and so far observational evidence supports the idea that moons can form regularly in such disks. There is no reason to believe we won’t find exomoons by the billions throughout the galaxy. Image: The University of...

A liquid water-defined habitable zone is a way of establishing parameters for life as we know it around other stars, and with this in mind, scientists study the amount of stellar radiation a planet receives as one factor in making the assessment. But of course, not everything in a habitable zone is necessarily habitable, as our decidedly uninhabitable Moon makes all too clear. Atmospheric factors and tectonic activity, for example, have to be weighed as we try to learn what the actual temperature at the surface would be. We're learning as we go about other contributing factors. A problem of lesser visibility in the literature, though perhaps just as crucial, is whether a given planet can stay habitable on timescales of billions of years. This is where an interesting new paper from Cayman Unterborn (Southwest Research Institute) and colleagues enters the mix. A key question in the view of these researchers is whether carbon dioxide, the greenhouse gas whose ebb and flow on our world...

What to make of a Jupiter-class planet that orbits its host star at a distance of 13.8 billion kilometers? This is well over twice the distance of Pluto from the Sun, out past the boundaries of what in our system is known as the Kuiper Belt. Moreover, this is a young world still in the process of formation. At nine Jupiter masses, it's hard to explain through conventional modeling, which sees gas giants growing through core accretion, steadily adding mass through progressive accumulation of circumstellar materials. Core accretion makes sense and seems to explain typical planet formation, with the primordial cloud around an infant star dense in dust grains that can accumulate into larger and larger objects, eventually growing into planetesimals and emerging as worlds. But the new planet - AB Aurigae b - shouldn't be there if core accretion were the only way to produce a planet. At these distances from the star, core accretion would take far longer than the age of the system to produce...

Exoplanet science can look forward to a rash of discoveries involving gravitational microlensing. Consider: In 2023, the European Space Agency will launch Euclid, which although not designed as an exoplanet mission per se, will carry a wide-field infrared array capable of high resolution. ESA is considering an exoplanet microlensing survey for Euclid, which will be able to study the galactic bulge for up to 30 days twice per year, perhaps timed for the end of the craft’s cosmology program. Look toward crowded galactic center long enough and you just may see a star in the galaxy's disk move in front of a background star located much further away in that dense bulge. The result: The lensing phenomenon predicted by Einstein, with the light of the background star magnified by the intervening star. If that star has a planet, it's one we can detect even if it's relatively small, and even if it's widely spaced from its star. For its part, NASA plans to launch the Roman space telescope by...

A living world around another star will not be an easy catch, no matter how sophisticated the coming generation of space- and ground-based telescopes turns out to be. It’s one thing to develop the tools to begin probing an exoplanet atmosphere, but quite another to be able to say with any degree of confidence that the result we see is the result of biology. When we do begin picking up an interesting gas like methane, we’ll need to evaluate the finding against other atmospheric constituents, and the arguments will fly about non-biological sources for what might be a biosignature. This is going to begin playing out as the James Webb Space Telescope turns its eye on exoplanets, and methane is the one potential sign of life that should be within its range. We know that oxygen, ozone, methane and carbon dioxide are produced through biological activity on Earth, and we also know that each can be produced in the absence of life. The simultaneous presence of such gases is what would intrigue...