Category Archives: Exoplanet formation

A second planet for WASP-107

WASP-107b is a hugely bloated planet, with a mass of only two Neptunes, but a radius near that of Jupiter, making it one of the puffiest planets known. As such it has been heavily studied, and indeed was the exoplanet showing the first detection of helium.

Long-term monitoring of the WASP-107 system with the Keck telescope has now revealed a companion planet, WASP-107c, as announced by Caroline Piaulet et al.

The new planet is in a much wider orbit, with a period of 1088 days and a high eccentricity of e = 0.26. It likely does not transit, and has a mass of perhaps a third that of Jupiter.

The discovery of a second planet is important for understanding the nature of WASP-107b itself. The tight, 5.7-d orbit, and the fact that the orbit is mis-aligned with the star’s rotation, might be explained by gravitational interactions with the second planet. The bloated size could then result from tidal interactions with the host star, as the planet circularised in its orbit, after the interactions with WASP-107c.

The authors conclude that, “Looking ahead, WASP-107b will be a keystone planet to understand the physics of gas envelope accretion”, starting with a planned observation with the soon-to-be-launched JWST.

Tidal inflation explains bloated exoplanet envelopes

A new paper by Sarah Millholland et al reconsiders highly bloated, low-mass planets such as WASP-166b. One explanation for the low mass of such planets is that they have small cores and are mostly gaseous envelope. However, having a relatively small core is at odds with core-accretion theory for the formation of such planets, which says that they can only gravitationally attract and then accrete large envelopes if the core is sufficiently massive.

Instead, Millholland et al suggest that the envelope is a smaller fraction of the planet’s mass than it seems, and that instead it has expanded to its current bloated state by tidal heating. A small eccentricity of the orbit is sufficient to produce tidal dissipation that heats the envelope and thus causes it to expand.

In the figure, the authors plot the fraction of the planet that is envelope, assuming no tidal heating, and also the smaller fraction when accounting for the effects of tidal heating. The reduction makes the proportions compatible with core-accretion theory. Millholland et al suggest that: “many sub-Saturns may be understood as sub-Neptunes that have undergone significant radius inflation, rather than a separate class of objects”.

Exoplanet formation scenarios

Here’s a nice graphic by Sean Raymond illustrating different scenarios for the formation of exoplanetary systems, one leading to “Super-Earths” and the other to gas giants. The work is explained more fully on arXiv.

The paper’s figure caption includes:

Left: Evolution of the “breaking the chains” migration model for the origin of super-Earths. Embryos within the snow line are entirely rocky and much smaller than those that form past the snow line, which also incorporate ice. Presumably ice-rich embryos migrate inward through the rocky material, catalyzing the growth of purely rocky planets interior to the ice-rich ones. Planets migrate into long chains of mean motion resonances, with the innermost planet at the inner edge of the disk. The vast majority (90–95%) of resonant chains become unstable when the gas disk dissipates. The resulting planets match the distributions of known super-Earths.

Right: Evolution of the planet-planet scattering model for the origin of giant exoplanets. Several embryos grow quickly enough to accrete gas and grow into gas giants. They subsequently migrate into a resonant chain without drastically affecting the orbits of nearby growing rocky planets (or outer planetesimal disks). After the disk dissipates, the vast majority (75–90%) of giant planets systems become unstable. The resulting systems match the correlated mass-eccentricity distribution of known giant exoplanets.