NASA’s Transiting Exoplanet Survey Satellite (TESS) is a satellite mission aimed at detecting planets around other stars using the transit method. A particular focus is small planets around stars smaller than the Sun. In late 2021 and late 2022, TESS observed a star called Gliese 12 and detected a possible transit of a small planet with a period of either 25.5 days or 12.76 days.
Together with some colleagues, we submitted a proposal to the European Space Agency (ESA) to use the CHEOPS satellite to resolve this period ambiguity. The proposal was lead by Larissa Palethorpe, a PhD student working with me. The proposal was successful, but it turned out that they’d also approved another proposal to look at the same target and that this proposal was also lead by a PhD student. We were encouraged to coordinate, which we did and managed to agree that the two PhD students would co-lead the work, which they did very well.
In the mean time, the period ambiguity was largely resolved by some ground based observations, and we also discovered that another group was analysing data from the same target. We made contact with this other group and agreed that we’d work independently and try to release papers at the same time. The other group were slightly ahead of us, but they were very accommodating and we managed to coordinate things quite well. This was all mostly done by Larissa, with me trying to be suitably encouraging.
The papers have just appeared and you can read ours here, and the other team’s paper here. With one exception, I think the author lists are completely independent. What makes this system particularly interesting, is that the planet has a radius of about 1 Earth radius and orbit its star at a distance that means it receives a flux between that received by the Earth and that received by Venus. It is, therefore, one of the most temperate small exoplanets detected to date.
However, if we define the habitable zone as the region where liquid water could persist on the surface, it is outside this zone and is in a zone where – if it has retained an atmosphere – a runaway greenhouse is quite likely. Also, it orbits a star that has a mass 24% that of the Sun, so is much closer to its star than the Earth is to the Sun. Therefore, it is probably tidally locked, with one side always facing the star. This will also influence how its atmosphere will evolve.
This doesn’t mean that it’s not a very interesting target. It’s worth studying further to see if it has retained an atmosphere, if there is any evidence for water, or – if there is still water – indications that water is escaping from the atmosphere. We wouldn’t really be doing this to try and infer if this planet were habitable, it would be to understand the composition, and evolution, of atmospheres of rocky planets around small stars. Also, the system is quite close to the Earth (12 parsecs) which makes it a good target for this kind of follow-up.
Because the initial detection was from NASA’s TESS mission, they were keen on a press release, which you can read here. I thought it was pretty well done, mostly comparing Gliese 12 b to Venus and highlighting how further study could help us understand “habitability pathways planets take as they develop.” However, quite a lot of the media coverage referred to it as “potentially habitable” which, even if we can’t completely rule this out, almost certainly isn’t going to be the case. Even if it has retained an atmosphere, it will probably be much more like Venus than the Earth. Maybe we could have been even more careful, but it’s probably impossible to completely avoid the media selecting the message that they think will appeal most to their readers. Some of the coverage was okay, though.
...and Then There's Physics 
Is the Gliese system a candidate for atmospheric spectroscopy using the caustic focusing of the central star’s light by transiting planetary limbs?
This has produced some fascinating results in our solar system:
Click to access Elliot1996AREPS.pdf
Russell,
Yes, it is a pretty good target for transmission spectroscopy with JWST.
Hey ATTP! Haven’t seen you post in a while—I dunno if that’s a fault of my notifications, or you actually haven’t. I’m the commenter who went by the username “Perse Show” something like 10 years ago, by the way.
Love this article, makes me want to take a course that includes exoplanet detection somewhere in the course of my astrophysics degree. And that’s coming from someone who generally leans toward stellar astrophysics and cosmology.
Emma,
Thanks. I haven’t posted for quite some time. A combination of running out of things to say, and being too busy 🙂
Good to see such co-operative science in action!
Dikran,
Indeed. It wasn’t all plain sailing, but it generally worked well and everyone cooperated nicely.
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” either 12.76 d or a factor of two larger (25.52 d), due to gaps in the photometry coincident with alternating transits.”
Is this what we would call a aliasing issue due to insufficient temporal sampling of the signal data?
Paul,
The way TESS works is that it stares at specific area of the sky for 27 days. So, if you had a system where the orbital period was 12.76 days, you’d expect to detect that because you should see 2 transits in a single visit. Also, it will often go back and look at that region again for another 27 days. In this case, it actually observed this target in 2 consecutive visits, so you would expect it to easily determine if the orbital period was 12.76 days because it observed the target for a total of 54 days.
However, in this case both the Earth and the Moon crossed the field of view during these observations, which results in quite large data gaps (many days) and those data gaps just happened to coincide with where you would expect the 12.76 day transits to lie. Hence, you couldn’t rule it out, or confirm it.
Thanks. Related to an aliasing error in that case. Astrophysicists have historically been keen in identifying these kinds of issues as I have gleaned from citations, such as inverting pulsar frequencies, etc . Most conventionally trained scientists think that frequencies as measured are simply related to as reciprocal wavelengths (in time or space depending on the measure). Unfortunately that is often not the case in the most perplexing of problems — such as where Brillouin zone folding occurs in situations arising within a periodic domain, including lattices (spatial folding) or periodically-induced fluid waves (temporal folding). Here the inversion problem is not as straightforward and modular arithmetic is needed to estimate the actual frequencies. The connection is that this folding is related to periodic signal aliasing. It’s likely at the heart of the difficulties I’ve had in trying to persuade others that many of the erratic climate oscillations are physical-aliasing inversion problems. This is simply related to the fact that there is a built-in annual or semi-annual (i.e. seasonal) cycle in climate behaviors, and any other periodic forcing applied to this will cause the equivalent of Brillouin zone folding and sub-banding. I guess I have not been running out of things to say, but running out ways to rephrase them until it hits home.