New Work on Planetary Inflation

Once in space in 2018, the Transiting Exoplanet Survey Satellite (TESS) will be observing, among many other things, hundreds of thousands of red giant stars across the entire sky. Planets around red giants are an interesting topic, because such stars point to an evolutionary outcome our own Sun will share, and we’d like to know more about what happens to existing planets in such systems as the host star swells and reddens, engulfing inner worlds.

New work out of the University of Hawaii Institute for Astronomy now examines two ‘hot Jupiters’ around red giants, stellar systems where we see the gas giants swelling up as the result of processes that remain controversial. The inflated size of planets like these can be explained in at least two ways, one of which involves a slowing of the cooling in the planet’s atmosphere, which causes the planet to inflate soon after formation. But the data presented here, drawn from NASA’s K2 mission, tend to corroborate the thinking of co-author Eric Lopez (NASA GSFC) that direct energy input from the host star is the dominant cause of this planetary inflation.

Image: Upper left: Schematic of the K2-132 system on the main sequence.

Lower left: Schematic of the K2-132 system now. The host star has become redder and larger, irradiating the planet more and thus causing it to expand. Sizes not to scale. Main panel: Gas giant planet K2-132b expands as its host star evolves into a red giant. The energy from the host star is transferred from the planet’s surface to its deep interior, causing turbulence and deep mixing in the planetary atmosphere. The planet orbits its star every 9 days and is located about 2000 light years away from us in the constellation Virgo. Credit: Karen Teramura, UH IfA.

The work is now available in The Astronomical Journal, where lead author Samuel Grunblatt and team show that each of the two planets is about 30 percent larger than Jupiter, though in each case only about half as massive. The two planets — K2-132b and K2-97b — are similar in orbital periods, radii and masses. Each orbits its red giant star in about nine days, with planetary radii being calculated at 1.31 ± 0.11 R J and 1.30 ± 0.07 R J respectively.

The researchers used models to analyze the evolution of planets like these over time, determining that their radii are typical for planets receiving their current level of radiation, but calculating back to main sequence values of radiation, they find the gas giants would have been considerably smaller. Stellar flux flowing to the planets’ deep convective interiors could therefore explain their current size, an indication that planet ‘inflation’ is directly tied to stellar irradiation rather than delayed atmospheric cooling after the planets’ formation.

But other factors remain to be tested, metallicity in particular. From the paper:

Further studies of planets around evolved stars are essential to confirm the planet re-inflation hypothesis. Planets may be inflated by methods that are more strongly dependent on other factors such as atmospheric metallicity than incident flux. An inflated planet on a 20 day orbit around a giant star would have been definitively outside the inflated planet regime when its host star was on the main sequence, and thus finding such a planet could more definitively test the re-inflation hypothesis. Similarly, a similar planet at a similar orbital period around a more evolved star will be inflated to a higher degree (assuming a constant heating efficiency for all planets). Thus, discovering such a planet would provide more conclusive evidence regarding these phenomena.

Also in play is the issue of heating efficiency, which may well vary between planets depending on their composition. Back to TESS, whose investigations should complement these results. Grunblatt and team point out that TESS should be able to observe additional planets in roughly 10 day orbits around more evolved stars, including oscillating red giants. The data should allow us to distinguish between the delayed cooling possibility and stellar irradiation scenarios.

The paper is Grunblatt et al., “Seeing Double with K2: Testing Re-inflation with Two Remarkably Similar Planets around Red Giant Branch Stars,” Astronomical Journal Vol. 154, No. 6 (27 November 2017). Abstract / preprint.