The Kepler-11 planetary system, with at least 6 planets in short period orbits. From https://en.wikipedia.org/wiki/Kepler-11.

Following on last week’s journal club where we discussed a paper in which collisions removed planetary atmospheres, this week we looked at a new paper by Aaron Boley and colleagues in which collisions promoted accretion of an atmosphere.

Boley and colleagues modeled gravitational interactions in tightly packed planetary systems, like the Kepler-11 system, 6 planets packed into a space smaller than Venus’ orbit.

Not surprisingly, when so many planets are packed into such a tight space, bad things can happen, and Boley and colleagues showed that such planets often collide with one another, sticking together to form even larger planets. In some cases, the newly formed planet can be large enough that it can accrete gas from its maternal protoplanetary disk and form a gas giant planet.

The standard model for planet formation suggests gas giants shouldn’t form close to their host stars, but Boley and colleagues argue that their collisional scenario could explain the presence of so many hot Jupiters and Neptunes found around Sun-like stars in the last few decades. Their work could help resolve the puzzle of hot Jupiters, an exoplanet mystery older than some of my students.

Journal club attendees included Jennifer Briggs, Emily Jensen, Karan Davis, Tyler Gordon, and Jacob Sabin. (Physics majors Jake and Steve also attended, but I don’t know their last names.)

Artist’s depiction of a collision between two planetary bodies. From https://en.wikipedia.org/wiki/Giant_impact_hypothesis.

We read a fun paper in journal club today, written by Inamdar and Schlichting of MIT that looks at the impact of large impactors on the atmospheres of gas-rich exoplanets.

Among the surprising discoveries of exoplanet searches is a huge class of  gas-rich planets between Neptune and Earth in size. Called sub-Neptunes or super-Earths, standard models for planet formation predict these planets shouldn’t exist — either they should have remained as small as the Earth as they accreted or they should have quickly grown to the size of Jupiter or Saturn. We don’t have planets like these in our solar system, but they may be one of the most abundant type of planet in the galaxy.

Even harder to understand, sub-Neptunes display a very broad range of densities, with some having densities greater than Earth’s and others with the density of wind-packed snow. This diversity indicates some planets have large rocky/icy cores with just a little gas on top, while others have tiny cores with bloated hydrogen/helium atmospheres. Since we think gaseous planets all form more-or-less the same way, it’s hard to explain this wide range of internal structures.

Inamdar and Schlichting explore the possibility that giant impacts between young planets in these systems could account for this diversity. By applying a simple 1-D hydrodynamic model, they show that these massively violent collisions could easily remove large amounts of atmosphere from the young planets.

Whether a certain planet experienced such a collision depends in a stochastic way on the initial conditions and gravitational interactions in these chaotic young planetary systems. So some planets would have experienced large collisions that removed a lot of their atmospheres, giving a high mean density, while others didn’t, leaving them low-density.

These same kind of planetary collisions shaped the diversity of planets in our own solar system. For example, the Earth’s Moon formed as the result of a collision between the proto-Earth and Mars-sized object, named Theia. Uranus probably got its unusual tilt from a collision with an Earth-sized object early in its history.

So even though most extrasolar planetary systems we know about don’t resemble our own, the results from this study show the same processes shaped them, and planets everywhere probably experienced a violent adolescence.

Journal club attendees today included Jennifer Briggs, Karan Davis, Hari Gopalakrishnan, Tyler Gordon, Emily Jensen, and Jacob Sabin.

Image of 51 Eri b (indicated by arrow) in the near-infrared, 1.65 microns.

A little behind the times on this one, but we finally managed to discuss the exciting discovery of a young, Jupiter-like planet in a Jupiter-like orbit by the Gemini Planet Imager (GPI) team at journal club today.

Using high-precision optical instrumentation and some sophisticated data processing to block out the host star’s glare, the team was able to directly image the planet 51 Eri b in infrared wavelengths.

No mean feat, given that the star is more than a million times brighter than the planet and is only one ten-thousandth of a degree away in the sky. This is a little like trying to see the glow of a firefly in the end zone against the glare of a football stadium light from the 50 yard line when the two are separated by the width of a human hair*.

51 Eri b is only 20 million years old, so it’s much hotter and glows much more brightly than our own Jupiter-like planets, making it easier to see. Jupiter-like planets tend to cool in a more-or-less well-behaved way that depends partially on their masses — bigger planets start out hotter. However, the planet is much cooler than predicted by some planet formation models, which provides strong constraints on the ways in which gas giants form.

So using 51 Eri b’s estimated temperature, 700 K (400 C) and age, Macintosh et al. put its mass somewhere between 2 and 12 Jupiter masses — solidly in the planet category.

The GPI observations also show us the planet has methane and water in its atmosphere. In fact, the methane detection for this planet is the most prominent so far seen for an exoplanet, according to Macintosh.

The GPI instrument is positioned to find many more planets like this one in the coming years, so expect lots of exciting results in the next few years.

Today’s journal club attendees included Jennifer Briggs, Hari Gopalakrishnan, Tyler Gordon, and Emily Jensen.

*Macintosh et al. estimate 51 Eri b’s luminosity is about 1 millionth that of our Sun. Wikipedia indicates the star 51 Eri is about 5 times brighter than our Sun. I had a lot of trouble finding the luminosity of a firefly — this page is the best I could do, and it estimates that one firefly emits about 2 mW. Stadium lights look to emit about 1,000 W, so that gives my factor of one million in luminosity.

51 Eri b has a projected separation from its host star of 13 AU, and the star is about 96 light years away, giving an angular separation of about 2 microradians. A human hair is about 100 microns across, so it would subtend 2 microradians from a distance of about 50 yards.