Among the discoveries announced at the recent meeting of the American Astronomical Society in Hawaii was TOI 700 d, a planet potentially in the habitable zone of its star. TOI stands for TESS Object of Interest, reminding us that this is the first Earth-size planet the Transiting Exoplanet Survey Satellite has uncovered in its data whose orbit would allow the presence of liquid water on the surface. The Spitzer instrument has confirmed the find, highlighting the fact that Spitzer itself, a doughty space observatory working at infrared wavelengths, is nearing the end of its operations. Thus Joseph Rodriguez (Center for Astrophysics | Harvard & Smithsonian):

“Given the impact of this discovery – that it is TESS’s first habitable-zone Earth-size planet – we really wanted our understanding of this system to be as concrete as possible. Spitzer saw TOI 700 d transit exactly when we expected it to. It’s a great addition to the legacy of a mission that helped confirm two of the TRAPPIST-1 planets and identify five more.”

Image: The three planets of the TOI 700 system, illustrated here, orbit a small, cool M dwarf star. TOI 700 d is the first Earth-size habitable-zone world discovered by TESS. Credit: NASA’s Goddard Space Flight Center.

TOI-700 is an M-dwarf star in the constellation Dorado, a southern sky object whose mass and size are roughly 40 percent that of the Sun, with half the Sun’s surface temperature. Remember that TESS monitors sky sectors in 27-day blocks, a period lengthy enough to spot the changes in stellar brightness that mark the transit of a planet across the star’s face as seen from Earth.

It’s interesting to note how any misclassification of stellar type can confound our conclusions about a transiting planet. In this case, the star was originally thought to be closer in size and type to the Sun, which would have meant planets that were larger and hotter than we now know are there. Correcting the problem revealed what looks to be a very interesting world.

“When we corrected the star’s parameters, the sizes of its planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone,” said Emily Gilbert, a graduate student at the University of Chicago. “Additionally, in 11 months of data we saw no flares from the star, which improves the chances TOI 700 d is habitable and makes it easier to model its atmospheric and surface conditions.”

Video: NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered its first Earth-size planet in its star’s habitable zone, the range of distances where conditions may be just right to allow the presence of liquid water on the surface. Scientists confirmed the find, called TOI 700 d, using NASA’s Spitzer Space Telescope and have modeled the planet’s potential environments to help inform future observations. Credit: NASA’s Goddard Space Flight Center.

What we now know about TOI 700 is that there are at least three planets here, with TOI 700 d being the outermost and the only one likely to be in the habitable zone. The planet is in a 37 day orbit and receives 86 percent of the insolation that the Sun gives the Earth. Here again we look to Spitzer, for its data allowed researchers not only to confirm the existence of TOI 700 d but also to tighten the constraints on its orbital period by 56% and its size by 38%. Further observations from the Las Cumbres Observatory network also tightened the orbital period.

The other worlds in the TOI 700 system are TOI 700 b, about Earth size and probably rocky, orbiting the star every 10 days, and TOI 700 c, 2.6 times larger than Earth and in a 16 day orbit. As to the intriguing TOI 700 d, let’s keep in mind that it’s relatively close at just over 100 light years, making it a potential target for follow-up observations by future space observatories, although not the James Webb Space Telescope, as I’ll explain in a moment. TOI 700 d is also likely to be, along with its planetary companions, in tidal lock with the star, keeping one side constantly in daylight, the other in perpetual night.

This nearby star appears to have low flare activity, adding to the potential that if TOI 700 d is truly in its habitable zone, any life developing there would not have to cope with severe doses of UV and X-rays. We should be able to get radial velocity information on this system that could firm up our assumptions about the composition of the three planets by determining their density when contrasted with the transit data that gives us their size.

For the time being, researchers at NASA GSFC have modeled 20 potential environments for TOI 700 d, using 3D climate models that consider various surface types and atmospheric compositions. Led by Gabrielle Engelmann-Suissa (a USRA visiting research assistant at GSFC), the team simulated 20 spectra for the 20 modeled environments. Dry, cloudless worlds and ocean-covered surfaces showed the range of possibilities. Such simulations can be of high value, as the paper on this modeling points out:

While the detection threshold of the spectral signals for this particular planet are most likely unfeasible for near-term observing opportunities, the end-to-end atmospheric modeling and spectral simulation study that we have performed in this work is an illustrative example of how global climate models can be coupled with a spectral generation model to assess the potential habitability of any HZ terrestrial planets discovered in the future, as we have done here with the exciting new discovery, TOI-700 d. With more discoveries on the horizon with TESS and ground-based surveys, we hope that this methodology will prove useful for not only predicting the observability of HZ planets but also for interpreting actual observations in the years to come.

The paper on the modeling is one of three describing the work on TOI 700 d. It makes clear that the noise floor of JWST, which takes into account instrument noise aboard the telescope, makes it unlikely the observatory will be able to characterize TOI 700 d. Similarly, direct imaging even by next-generation extremely large telescopes (ELTs) is challenging. Thus the paper’s conclusion: “Significant characterization efforts will therefore require future space-based IR interferometer missions such as the proposed LIFE (Large Interferometer For Exoplanets) mission.”

It’s good, then, to have TOI 700 d in our catalogs, but it’s not going to be the first exoplanet whose atmosphere we can probe for potential biosignatures.

Three papers describe this work. They are Gilbert et al., “The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System,” submitted to AAS Journals (preprint); Rodriguez et al., “The First Habitable Zone Earth-Sized Planet From TESS II: Spitzer Confirms TOI-700 d,” submitted to AAS Journals (preprint); Engelmann-Suissa et al., “The First Habitable Zone Earth-sized Planet from TESS. III: Climate States and Characterization Prospects for TOI-700 d,” submitted to the Astrophysical Journal (preprint).