NASA’s planet-hunting telescope, the Transiting Exoplanet Survey Satellite (TESS), has detected a second Earth-sized planet within the habitable zone of a nearby star.
Known as TOI 700 e, the planet is one of four known planets orbiting a cool star about 100 light-years away. The system was already known to host one planet, called TOI 700 d, in the habitable zone, but Recent research to be published in Astrophysical Journal Letters He reveals that he has joined another planet within its orbit. The other two planets in the system, TOI 700 b and TOI 700 c, orbit closer to the star and, as such, are likely to have higher temperatures, which puts them outside the habitable zone.
One of the researchers, Emily Gilbert of NASA’s Jet Propulsion Laboratory, joked in a briefing at the American Astronomical Society meeting on Tuesday, January 10. Planets are given letters according to the date they were discovered rather than their location within a system, so there can be cases like this when planets in closer orbits are discovered later than planets in more distant orbits.
The recently discovered planet TOI 700 e is located in a region defined as the optimistic habitable zone, while the previously discovered planet TOI 700 d is within a region called the habitable zone. The traditional definition of a habitable zone is an area around a host star where temperatures are such that liquid water can exist on a planet’s surface. However, this definition is more complicated to apply in practice than it might seem—which is why these researchers use the terms “optimistic” and “conservative.”
“I’m so sorry they’re not in alphabetical order.”
The optimistic habitable zone indicates an area where liquid water could exist at some point In the history of a planet, while the conservative habitable zone is a smaller region within that in which planets will remain habitable. These two differ due to the planet’s surface temperature – and therefore whether water can exist in liquid form – and can vary widely based on factors such as the thickness and composition of the planet’s atmosphere over time.
This expansion of the traditional habitable zone “explains the fact that we think Mars and Venus had liquid water on their surfaces,” Gilbert explained, pointing to evidence of water on both planets billions of years ago. Studying planets within this optimistic region expands the number of planets that astronomers can use to understand the history of our solar system.
Astronomers can also compare the four planets within the TOI 700 system with each other. “We know that these planets formed under the same initial conditions — they formed around the same star, from the same disk. This enables us to study how different planet features can affect a planet’s habitability,” Gilbert said, including such features as planet size or habitable zone boundaries.
TOI 700 e is located in an area defined as the Optimistic Habitable Zone
This system is one of the few that we know of 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 rocky Earth-sized planet and the first exoplanet discovered by the James Webb Space Telescope, or JWST. However, this 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, the first indication of a possible exoplanet being identified by TESS before it was confirmed by the JWST.
We can expect more exoplanet results from both telescopes in the future, and the research team for this TESS discovery says they will continue follow-up studies of the TOI 700 system to learn more about exoplanets.