Read on Twitter
Woop woop! Our paper on three planets orbiting L231-32 is out!
We measured the masses and compositions of its three small planets, and found some exciting features related to the exoplanet radius valley that helps us understand their compositions.
Paper: https://arxiv.org/abs/2101.01593
We measured the masses and compositions of its three small planets, and found some exciting features related to the exoplanet radius valley that helps us understand their compositions.
Paper: https://arxiv.org/abs/2101.01593
What did we do?
The NASA @TESSatMIT satellite saw three planets around L231-32, a nearby small star also known as TOI-270. TESS measured the brightness (flux) of this star over time, and recurring dips when these 3 planets (b, c, d) cross (transit) in front of the star are seen.
The NASA @TESSatMIT satellite saw three planets around L231-32, a nearby small star also known as TOI-270. TESS measured the brightness (flux) of this star over time, and recurring dips when these 3 planets (b, c, d) cross (transit) in front of the star are seen.
Based on the spacing between these transits and their depth, we know that these planets are similar in size to Earth, at 1.1, 2.3, and 2.0 times its size. Their orbits are short, with periods of 3.4, 5.7, and 11.4 days.
We then observed this same star using the @ESO HARPS instrument and its super-state-of-the-art ESPRESSO instrument. These instruments use the Doppler shift measure the wobble of the star caused by the planets, known as the star's radial velocity (RV). It's a few meter per second!
I repeat: the star wobbles back and forth at a few meter per second. You can walk faster than that!
With @eso's ultra-precise spectrographs HARPS & ESPRESSO, we can measure this tiny movement in a star 100 light year away.
We then see the effect of each of the three planets.
With @eso's ultra-precise spectrographs HARPS & ESPRESSO, we can measure this tiny movement in a star 100 light year away.
We then see the effect of each of the three planets.
This periodic wobble is a gravitational effect, which we use to derive the planet masses. They are about 1.6, 6.1, and 4.8 Earth masses. Since we already learned the planet sizes from the TESS transits, we now have both the mass and size of these planets. Here's how it looks.
Let's look at the smallest one first, planet L231-32b.
It's *really* small. Only a handful of smaller planets have reliable mass estimates. What's more, all 7 smaller such planets orbit one star: Trappist-1.
Small planets all seem to be 'rocky', with a composition like Earth.
It's *really* small. Only a handful of smaller planets have reliable mass estimates. What's more, all 7 smaller such planets orbit one star: Trappist-1.
Small planets all seem to be 'rocky', with a composition like Earth.
Not so much for L231-32c and L231-32d. These planets are quite a bit larger, but not that much more massive: they have a lower density.
We think this is because they have an atmosphere of water and helium, which takes up a lot of *space* but is very *light*.
We think this is because they have an atmosphere of water and helium, which takes up a lot of *space* but is very *light*.
So why do L231-32c and L231-32d have a H-He atmosphere, but doesn't L231-32b?
We think L231-32b formed with a similar atmosphere, but quickly lost it, possibly due to photo-evaporation as this planet is so very close to its star and receives a lot of stellar radiation.
We think L231-32b formed with a similar atmosphere, but quickly lost it, possibly due to photo-evaporation as this planet is so very close to its star and receives a lot of stellar radiation.
Inferring a composition from mass + radius is notoriously challenging, as often multiple combos of planet cores + atmospheres give same answers.
But: if all three planets have the same rocky core, one lost its atmosphere and the other two didn't, that explains the data.
But: if all three planets have the same rocky core, one lost its atmosphere and the other two didn't, that explains the data.
Could it be that the outer two planets have a very low-density core instead? Perhaps. But for three planets in near-resonant orbits, it seems more likely that their core compositions are similar. In that case, planet c and d need an atmosphere, planet b does not.
This is also what some models (e.g. photo-evaporation) would predict. Small, super-Earth planets have had their atmospheres stripped away. Larger, sub-Neptune planets held on their H-He. In between super-Earths and sub-Neptunes is a valley, an absence of planets.
We looked at all planets orbiting small stars for which we reliably know both mass and radius.
They split into two groups, separated by a planet radius valley!
And, excitingly, sub-Neptunes above the valley seem to possess a H-He atmosphere, whereas super-Earths below don't!
They split into two groups, separated by a planet radius valley!
And, excitingly, sub-Neptunes above the valley seem to possess a H-He atmosphere, whereas super-Earths below don't!
There are plenty of caveats. Firstly, these are low-number statistics, i.e. very few small planets have reliable M+R. Secondly, these stars, while all small, have a wide mass range. Thirdly, two planets on the plot don't fit the simple picture. Fourthly, M-R degeneracies.
Nevertheless:
1/ Small planets orbiting small stars seem to split into two categories based on their period and radius.
2/ Their masses then show that one category has a H-He atmosphere, whereas the other does not (anymore).
This broadly matches theoretical predictions.
1/ Small planets orbiting small stars seem to split into two categories based on their period and radius.
2/ Their masses then show that one category has a H-He atmosphere, whereas the other does not (anymore).
This broadly matches theoretical predictions.
Finally, if you're a person interested in studying planetary atmospheres with instruments like @NASAWebb, these planets are also *great* targets for such studies! That could tell us a lot more about these H-He atmospheres (or lack thereof).
Details in the preprint of our submitted (not yet accepted) paper are here: https://arxiv.org/abs/2101.01593 . Feedback very welcome!
My many thanks to colleagues at @Princeton @PU_Astro, where I started this work, and at @ucl @MSSLSpaceLab @msslastro where it was ultimately completed! :)
My many thanks to colleagues at @Princeton @PU_Astro, where I started this work, and at @ucl @MSSLSpaceLab @msslastro where it was ultimately completed! :)
Finally, my gratitude to a very large and truly international team, who have made all of this possible!
Including: @astrodillo @kristinewflam @justesen @exobeatriz @exoplaneteer @AstronomerMac @hlmosborne @oscaribv @dacmess @ExoCharbonneau @rene_doyon @ProfSaraSeager @twitspek
Including: @astrodillo @kristinewflam @justesen @exobeatriz @exoplaneteer @AstronomerMac @hlmosborne @oscaribv @dacmess @ExoCharbonneau @rene_doyon @ProfSaraSeager @twitspek
