Two Earth-sized habitable zone planets in the same system!

Title: A Second Earth-Sized Planet in the Habitable Zone of the M Dwarf, TOI-700

Authors: Emily A. Gilbert, Andrew Vanderburg, Joseph E. Rodriguez, et al.

First Author’s Institution: NASA Jet Propulsion Laboratory, California Institute of Technology, Pasadena, California, USA.

Status: Published in The Astrophysical Journal Letters [open access]

TESS and Dr. Emily Gilbert have done it again! Three years ago, Dr. Gilbert, then a graduate student, and her team found the first Earth-sized planet in the habitable zone of a TESS exoplanet system called TOI-700. Planets in habitable zones are close enough to their host planets to harbor liquid water, and are some of the best places to consider in the search for life. Now, analysis of new TESS data has yielded another Earth-sized habitable zone planet, hidden in the same TOI-700 system! 

The host star of TOI-700 is an M-dwarf star. M dwarfs are some of the smallest and most plentiful stars in our galaxy –  astronomers estimate they may account for up to 75% of the stars in the Milky Way. Because they are so small, M dwarfs are extremely long lived, far exceeding the 10 billion-year lifetimes of stars like our Sun. Astronomers have been interested in planets around M dwarfs for years now. The small size and low brightness of M-dwarfs make it easier to detect planets around these stars, meaning M-dwarfs are a great place to start to search for habitable zone planets. 

TESS finds exoplanets using the transit method, monitoring stars for the tell-tale brightness variations that occur when planets pass in front of their host stars (you can help astronomers find transiting TESS planets here!). The TOI-700 system was observed multiple times in the first three years of TESS’ mission, and the first analysis of TOI-700 yielded three planets, referred to as b, c, and d (the star itself typically gets ‘a’). Those initial three planets all had radii between 1-2 Earth radii. Planets b and c are orbiting very close to the host star, with orbital periods of less than 16 days. However planet d, with a period of ~40 days, falls within the habitable zone for the star TOI-700!

Figure 1: Lightcurves for the four TOI-700 planets. Each plot shows the change in observed brightness of the host star when the planet passes in front of the star. Planets d and e are both in the habitable zone of the system! Credit: Figure 3 from today’s paper

Today’s authors used existing TESS data, along with new observations at the Campocatino Austral Observatory (CAO) in Chile, to discover yet another habitable zone planet in the TOI-700 system, planet e! The CAO data was taken to make sure no binary stars were interfering with the observed light curves. 

The transit data show that planet e is actually a little closer to the host star than planet d, technically placing planet e in what is called the “optimistic” habitable zone (Figure 2). The difference between the “optimistic”  and “conservative” habitable zones is small, but the distinction is a sign of our uncertainty on what constitutes habitability. While we define the habitable zone as the range of radii away from a host star where planets can sustain liquid water, calculating those radii can vary based on our assumptions about where liquid water exists in the solar system. A conservative estimate of the habitable zone is calculated based on maximum greenhouse effects that could apply to a planet, whereas the optimistic zone is based on estimating the history of liquid water on Venus and Mars. 

Figure 2: Diagram of the TOI-700 system, with the newly discovered planet e in red. The darker green region is the “conservative” habitable zone while the lighter green is the “optimistic” zone. Credit: Figure 4 from today’s paper

The radius of a planet can be determined based on the depth of the transit light curve, since the depth corresponds with the ratio of the cross section of the planet relative to the size of the host star. Planet e has a radius of about 0.95 Earth radii, and the authors found that the planet is most likely rocky, with a mass of about 0.85 Earth masses. From that mass, the authors were able to further estimate that the planet would have become tidally locked with the host star within a few million years of its formation, remaining so to the present day. 

So what does it mean to have two habitable zone planets in one system? In short, it’s an exceptional opportunity to learn about habitability and exoplanet climates! The TOI-700 planets all have similar masses and radii to the Earth, so they provide a really good opportunity to compare the terrestrial planets in our solar system to exoplanets overall. Very few detected planetary systems have multiple low-mass planets inside or outside of the habitable zone. The notable existing example is the TRAPPIST system, and it will be really interesting to compare the two systems going forwards. The TRAPPIST host star is much more active than TOI-700, so together these two systems could shed light on how different stellar properties affect planets. 

The TOI-700 planets are also all on nearly circular orbits, meaning they are more likely to have long-term, stable climates than more eccentric planets. The brightness of the TOI-700 host star is sufficient for observing the atmospheres of the planets in the future, so we could learn a lot more about planetary climates and how they can vary throughout solar systems. 

Finally, planet e is especially compelling because it receives a stellar flux from the host star that’s similar to what Earth and Venus get from the Sun. So planet e could be a really interesting Earth/Venus analog and maybe even help us understand whether Venus had liquid water, and if so what happened to it. 

The discovery of TOI-700’s newest planet is just the tip of the iceberg for this system! TESS is planning to observe the system again in an upcoming observing cycle, to check for any more hidden planets. The authors are also gathering radial velocity data for the system to get better constraints on the masses of the planets. Keep an eye out for future TOI-700 results, because there’s so much exciting science to do with this system! 

Astrobite edited by: Sarah Bodansky

Featured Image Credit: NASA/JPL-Caltech/Robert Hurt

About Isabella Trierweiler

I'm a fifth year grad student at UCLA. I'm interested in planet formation and I study the compositions of exoplanets using polluted white dwarfs. In my free time I like knitting, playing train games, and growing various fruit trees.

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