The news of a gas giant of half Jupiter's mass around a small red dwarf, GJ 3512 b, continues to resonate. It goes to what has become a well enshrined controversy among those who follow planet formation models. While core accretion is widely accepted as a way of building planets, gravitational instability has remained an option. We are not talking about replacing one model with another, but rather saying that there may be various roads to planet formation among the gas giants. In any case, GJ 3512 b makes a strong case that we have much to learn. When I think about gravitational instability, I go back to the work of Alan Boss (Carnegie Institution for Science), as he has long investigated the concept. I learned about it from his papers and his subsequent book The Crowded Universe (Basic Books, 2009). Here's how Boss describes it there: Proponents of the top-down mechanism… envision clumps of gas and dust forming directly out of the planet-forming disk as a result of the...

The gas giant GJ 3512 b does not particularly stand out at first glance. About 30 light years from the Sun, it orbits its host star in 204 days, discovered by radial velocity methods by the CARMENES collaboration, which is all about finding planets around small stars. But look more deeply and you discover what makes this find provocative. GJ 3512 b turns out to be a gas giant with about half the mass of Jupiter, and small red dwarfs like this one aren’t supposed to host such worlds. In fact, GJ 3512 b is at least an order of magnitude more massive than what we would expect from current theoretical models, making it an interesting test case for planet formation. Core accretion models assume the gradual agglomeration of material in a circumstellar disk, with small bodies banging into each other and growing over time until their gravity is sufficient to draw in an atmosphere from the surrounding gas. This gas giant defies the model, evidently having formed directly from the disk through...

Ongoing in Geneva is the joint meeting of the European Planetary Science Congress and the Division for Planetary Sciences of the American Astronomical Society. We can abbreviate the whole thing as EPSC-DPS 2019, and you can read more about it here. We'll track several stories here as they develop, but I notice that the European Space Agency's ARIEL mission, which is slated to make the first large-scale survey of exoplanet atmospheres, has been supporting a Data Challenge involving removing noise from exoplanet observations. So let's start there. The slant here is training computers to filter out errors in collecting exoplanet data caused by starspots and by instrumentation, with two winners, James Dawson (Team SpaceMeerkat), and Vadim Borisov (Team major_tom), announced yesterday in Geneva. All told, 112 teams registered for the competition, a heartening number illustrative of the growing interest in computational statistics and machine learning among exoplanet researchers. The top...

We're beginning to probe the atmospheres of planets other than gas giants, a step forward that the next generation of space- and ground-based instruments will only accelerate. This morning we have word that the habitable zone 'super-Earth' eight times as massive as Earth orbiting the star K2-18 has been found to have water vapor in its atmosphere, making it the only exoplanet known to have water as well as temperatures that could sustain that water as a liquid on the surface. This is also our first atmospheric detection of any kind for a planet orbiting in the habitable zone of its star. Angelos Tsiaras (University College London Centre for Space Exochemistry Data) is lead author on this work, which appears today in Nature Astronomy: "Finding water in a potentially habitable world other than Earth is incredibly exciting. K2-18b is not 'Earth 2.0' as it is significantly heavier and has a different atmospheric composition. However, it brings us closer to answering the fundamental...

Our thinking on how planetary systems form includes the accretion of rocky bodies within a disk surrounding a young star, and we're examining such disks in numerous systems, such as the well studied Beta Pictoris. But the idea of accretion leaves many issues unsettled, such as what happens when large rocky bodies collide in the violent endgame of system formation. The Earth evidently underwent such a collision, with our own Moon being the tangible result. Caltech postdoc Simon Lock has been working with Sarah Stewart (UC-Davis) to study how such giant impacts unfold, running simulations of early planetary materials whose collisions can form bodies with masses between 0.9 and 1.1 Earth masses. The energy involved in such impacts is thought to allow, in some cases, the two colliding bodies to form a 'synestia,' or a rotating torus of planetary materials that will later cool into one or more spherical planets. The synestia is, however, but one outcome out of many produced by these...

It shouldn’t surprise us that first discoveries can be extreme. Consider that the first main sequence exoplanets we detected were ‘hot Jupiters.’ Nobody expected these (unless you discount John Barnes and Buzz Aldrin in Encounter with Tiber, and Greg Matloff, who advised them -- see Probing Ultrahot Jupiters -- but a radial velocity detection is rendered far more likely if a large planet is orbiting close to its star. And so we got 51 Pegasi b, and soon, others in the hot Jupiter category. Incidentally, the Barnes & Aldrin novel was finished though not published when the discovery of 51 Pegasi b was made in 1995. Nice prediction! Hot Jupiters may not be all that common, but they show up in early radial velocity work. I could throw in the first exoplanets of another kind as well, these being planets around a pulsar. Who, as Isidor Rabi once said about muons, ordered that? Extreme objects that push hard enough on their environment to be flagged by our current instrumentation are of...