Using a revolutionary NASA telescope, astronomers have discovered a remarkably Earth-like world orbiting in the habitable zone of a distant red dwarf star.
Artist mock-up. TOI 700, a planetary system 100 light-years away in the constellation Dorado, is home to TOI 700 d, the first Earth-size habitable-zone planet discovered by NASA's Transiting Exoplanet Survey Satellite. Image Credit: JPL/NASA's Goddard Space Flight Center
Excitingly, this isn’t the first extrasolar planet, or exoplanet, discovered around this star, designated TOI-700, and neither is it the first planet found located within this system’s habitable zone, defined as the region in which liquid water can exist.
The findings which have been accepted for publication by The Astrophysical Journal Letters are available on the paper repository ArViX and were announced by research team leader and NASA Jet Propulsion Laboratory post-doc, Emily Gilbert, at the 241st meeting of the American Astronomical Society in Seattle.
To understand why the discovery of this exoplanet, TOI-700 e, is exciting, it’s necessary to explore both the qualities of this planet, its companion planets, the system in which it dwells, and the circumstances under which it was discovered.
What do we know about TOI-700 e?
Discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS), TOI-700 e is the fourth planet discovered in this particular star system, which is located around a red dwarf 100 light-years away from Earth.
TOI-700 e, for which the prefix TOI means “TESS Object of Interest,” is around 95% the size of Earth. On the basis of this size, the research team believes the exoplanet is very probably a rocky or terrestrial world like our planet. The newly discovered exoplanet orbits its star in just 28 days.
As the fact that Earth’s year is over 13 times longer than that of TOI-700 e implies, this newly discovered exoplanet is much closer to its star than our planet is to the sun. Despite this proximity, TOI-700 e is still considered to be located within the habitable zone of its red dwarf parent star. This is because TOI-700 is only 40% as large as the sun and also has around 40% of our star’s mass.
This means that the dimmer star TOI-700 has a surface temperature that is much cooler than the sun’s, at just around 3500 Kelvin compared to the sun’s surface temperature of around 6000 K. As a result, planets can exist much closer to TOI-700 e without their surface water boiling away than they could to a star as hot as the sun.
TOI-700 e isn’t alone in its star’s habitable zone either, making this red dwarf-centered system one of just a few collections of planets we are aware of that have multiple possibly habitable worlds.
A red dwarf and its habitable planets
This is the fourth planet discovered in the TOI-700 system. The previous planets, TOI-700 b, TOI-700 c, and TOI-700 d, were all also discovered by TESS. Their discovery was announced in 2020 in a paper also authored by Gilbert and her team.
Of these planets, TOI-700 d is significant because it is also believed to be a terrestrial world roughly the same size as Earth, this time 1.1 times the size of our planet.
The innermost planet of the system, TOI 700 b, is around 90% the size of our planet Earth’s size and is so close to the red dwarf parent star that it completes an orbit every ten days. TOI 700 c is the largest planet discovered in the system thus far, at 2.5 times the size of Earth.
Both TOI 700 b and TOI 700 c are so close to their star that they are probably tidally locked. This means as they orbit TOI 700 one side of these planets permanently faces the red dwarf. This is a situation that can give rise to intense temperatures at the star-facing “dayside” of a planet.
TOI-700 d is significant as it happened to be the first habitable zone terrestrial planet spotted by TESS, a telescope set to have a major impact on our investigation of exoplanets.
TESS and the future of exoplanet investigation
TESS launched into space aboard a SpaceX Falcon 9 rocket from Cape Canaveral on April 18, 2018, and it spots planets as they cross, or transit, the face of their star.
Dividing the sky into segments called continuous viewing zones (CVZs) monitors over 200,000 stars looking for these telltale dips that indicate the presence of a planet. From its highly elliptical cislunar orbit around Earth, TESS studies each of these segments for a 27.4-day observational period. This makes it particularly adept at spotting short-orbit planets with periods of less than 13 days, that exist close to their star and cross the face of it twice during an observational period.
The team behind TESS expected at launch to discover thousands of exoplanets. Among these exoplanets, the team has committed to discovering at least 50 planets with sizes less than four times the size of Earth.
The discovery of TOI-700 b, TOI-700 c, and TOI-700 d was announced in 2020 by Gilbert and came from the first year of observational data from TESS. TOI-700 e proved to be a tougher customer to confirm, however.
Because this newly found exoplanet was further out from its star, the dip in light it causes as it transits TOI-700, and thus the signal indicating its presence, was faint.
TOI-700 e is located in what scientists call the “optimistic habitable zone”, the range at which liquid water could have existed on a planet’s surface at some point in its history. This is different from what is known as the “conservative habitable zone”, at which point liquid water could exist throughout a planet’s existence. TOI 700 d, the Earth-like companion of this newly found exoplanet, orbits in this latter region.
While researchers search for other similar planetary systems with multiple Earth-like worlds using TESS, astronomers aren’t dome with TOI-700 and its planets.
The system is still being studied by both space and ground-based observatories, with scientists hoping to gain more insights into this system and thus the solar system and its place in the universe.
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References and Further Reading
E. A. Gilbert., A. Venderberg., J. E. Rodriguez., et al, “A Second Earth-Sized Planet in the Habitable Zone of the M Dwarf, TOI-700,” [2023], [https://arxiv.org/pdf/2301.03617.pdf]
E. A. Gilbert., T. Barclay., J. E. Schlieder., et al, The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System, [2020], [https://arxiv.org/abs/2001.00952]
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