NASA’s planet-seeking telescope, the Transiting Exoplanet Survey Satellite (TESS), has discovered a second Earth-sized planet within the habitable zone of a nearby star.
Named TOI 700 e, the planet is one of four known planets orbiting a cool star about 100 light-years away. It was already known that the system harbors one planet, called TOI 700 d, in the habitable zone, but recent research that will be published in the Astrophysical Journal Letters reveals that it is joined by another planet in its orbit. The other two planets in the system, TOI 700 b and TOI 700 c, orbit closer to the star and are therefore likely to have higher temperatures, moving them outside the habitable zone.
The new planet “is right there between planets c and d, so I’m very sorry they’re not in alphabetical order,” joked one of the researchers, Emily Gilbert of NASA’s Jet Propulsion Laboratory, during a briefing at the American Astronomical Society meeting. on Tuesday, January 10. Planets are lettered based on their date of discovery rather than their position within a system, so there may be cases like this when planets orbiting closer together are discovered later than planets orbiting farther.
The recently discovered planet TOI 700 e is in an area defined as the optimistic habitable zone, while the previously discovered planet TOI 700 d is in an area called the conservative habitable zone. The traditional definition of the habitable zone is an area around a host star where the temperature is high enough that liquid water could exist on a planet’s surface. However, this definition is more complicated to apply in practice than it seems – which is why these researchers use the terms ‘optimistic’ and ‘conservative’.
“I am very sorry that they are not in alphabetical order”
The optimistic habitable zone refers to an area where liquid water could have been present at a given moment in a planet’s history, while the conservative habitable zone is a smaller region within which planets would remain habitable. These two are different because of a planet’s surface temperature — and thus whether water could exist in liquid form — and can vary widely based on factors such as the thickness and composition of a planet’s atmosphere over time.
This extension of the traditional habitable zone is “to account for our belief that Mars and Venus once had liquid water on their surfaces,” Gilbert explained, referring to evidence that there was water on both planets billions of years ago. Studying planets within this optimistic zone increases the number of potentially habitable planets that astronomers could use to understand the history of our own solar system.
Astronomers can also compare the four planets within the TOI 700 system. “We know that these planets formed under the same initial conditions — they formed around the same star, from the same disk. So this allows us to study how different planet properties might affect the planet’s habitability,” said Gilbert, including properties such as the size of the planet or the limits of the habitable zone.
TOI 700 e is located in an area defined as the optimistic habitable zone
This system is one of the few known to host multiple Earth-sized planets within its habitable zone, joining famous systems such as the TRAPPIST system. The announcement of the discovery also came the day before the announcement of LHS 475 b, another Earth-sized rocky planet and the first exoplanet to be discovered by the James Webb Space Telescope, or JWST. However, that planet is much closer to its star and is outside the habitable zone. The TESS and JWST missions worked together to identify this new exoplanet, as an initial indication of a potential exoplanet was flagged by TESS before being confirmed by JWST.
We can expect more exoplanet findings from both telescopes in the future, and the research team for this TESS discovery says they will continue with follow-up studies of the TOI 700 system to learn more about its exoplanets.