So excited to share a brand new result at #ExSS4! Here's a thread summarizing my talk for all you astrotweeps :). the paper will be out on arxiv tonight.
tldr: We measured a thermal phase curve of a terrestrial exoplanet for the first time. Shout-out to my coauthors, particularly the theory dream team: Daniel Koll, Caroline Morley @AstroCaroline, Renyu Hu, and Laura Schaefer @lavainspace
The question we wanted to answer was : "how often do terrestrial planets keep their atmospheres?"
even in the Solar System, there's a big range, from Mercury's femtobar atmosphere to 100 bars of CO2 on Venus. but it's hard to generalize from this sample. most terrestrial planets in the Galaxy orbit small M-dwarfs, not Sun-like stars!
this is exciting, because these M-dwarf planets are relatively easy to characterize with transmission/emission spectra (thanks to the large planet/star radius).
but the planets' atmospheres are in danger! M-dwarfs are super bright in the UV for the first billion years of their lives, which can photoevaporate the whole atmosphere. later on, flaring and wind can further erode any atmosphere that is left :(
observations so far: transmission spectra of the TRAPPIST-1 planets and GJ 1132b rule out H-rich, cloud-free atmospheres. but the data are consistent with high mean molecular weight compositions, cloudy atmospheres, or no atmosphere at all
so is there anything else we can measure to help us figure out if an atmosphere is there?
YES: a thermal phase curve!! the idea is that thin atmospheres are bad at redistributing heat. Large phase variation -> large temperature variation -> thin/no atmosphere.
aficionados will remember this technique was applied to 55 Cancri e, a super-Earth that is so large (1.95 Earth radii) that it likely has some atmosphere. the phase curve had an offset hotspot, suggestive of atmospheric heat circulation to the nightside! (Demory et al. 2016)
but until now, no terrestrial planet has been accessible for phase curve observations. enter LHS 3844b, discovered by @NASA_TESS -- a 1.3 Earth radius world orbiting a nearby M-dwarf with an 11-hr orbital period
luckily for us, this planet is hot enough to detect its thermal emission with @NasaSpitzer !! we observed the system for 100 continuous hours
This is what we got: 😮😮😮 
what does this mean???? well: a couple quick takeaways. the dayside temperature is 1080 +/- 40 K, and the nightside is consistent with 0 K (1 sigma). there is no significant hotspot offset.
this looks an awful lot like a bare rock, but we did our homework and asked : is there any atmosphere that is consistent with the data?
here is the measured eclipse depth compared to theoretical models from Daniel Koll and @AstroCaroline. we predicted the eclipse depths for O2 + CO2 atmospheres, a likely outcome for hot terrestrial planets. surface pressures greater than 10 bar are ruled out!!
Thin atmospheres (< 1 bar) *are* consistent with the data, but are these stable? Atmosphere evolution models from @lavainspace suggest not:
these models show the final atmospheric pressure for models starting with an initial H2O envelope (O2 is left over after gigayears of photoevaporation).
there are finely-tuned scenarios that produce a 1 - 10 bar atmosphere, but stellar wind is expected to fully erode that atmosphere over the planet's lifetime.
so: we are most likely observing the rocky surface of LHS 3844b. What kind of rock might it be?
rocky surface models from Renyu Hu show that basaltic compositions are the best fit to the data. These rocks could form from cooling lava, similar to the lunar mare or the surface of Mercury (or the black sand beaches in Iceland!!)
back to our original question: how often do terrestrial planets maintain atmospheres? from a sample of one, it looks like hot planets around M-dwarfs may have trouble.
but thanks to new detections from @NASA_TESS and the extraordinary capabilities of @NASAWebb, we can observe more! maybe cooler planets are more likely to have atmospheres. we shall see!!!!!
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