NASA planet hunter TESS spots its first Earth-size exoplanet in habitable zone

Discovered by the TESS and dubbed TOI 700 d, it is one of the few Earth-size planets found in potentially habitable zones.


                            NASA planet hunter TESS spots its first Earth-size exoplanet in habitable zone
TESS (NASA Goddard Space Flight Center)

NASA's planet hunter, the Transiting Exoplanet Survey Satellite (TESS), has disocvered its first Earth-size world in its star's habitable zone. This means that the planet called TOI 700 d has the potential for liquid water on its surface.

TOI 700 is a small, cool M dwarf star located just over 100 light-years away in the southern constellation Dorado. The star appears in 11 of the 13 sectors TESS observed during the mission's first year, and scientists caught multiple transits by its three planets.

“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,” says NASA in a statement.

TOI 700 d is one of the few Earth-size planets found in a star’s habitable zone. Others include several planets in the TRAPPIST-1 system and other worlds discovered by NASA's Kepler Space Telescope.

"TESS was designed and launched specifically to find Earth-sized planets orbiting nearby stars. Planets around nearby stars are easiest to follow-up with larger telescopes in space and on Earth. Discovering TOI 700 d is a key science finding for TESS. Confirming the planet's size and habitable zone status with Spitzer is another win for Spitzer as it approaches the end of science operations this January," says Paul Hertz, astrophysics division director at NASA Headquarters in Washington.

In a galaxy, 100 light-years away in the constellation Dorado, sits a planetary system named TOI 700. It is home to TOI 700 d. (NASA/Goddard Space Flight Center)

The researchers presented the findings at the 235th meeting of the American Astronomical Society in Honolulu, and three papers have been submitted to scientific journals.

According to the team of scientists, TESS stares at patches of sky for long stretches, recording light from thousands of stars. Some of these stars have planets that cross or transit in front of them. TESS sees these events as tiny, regular dimmings of host stars. 

One star where TESS saw transits is TOI 700. It is a red dwarf, about 40% of the mass and size of our Sun and roughly half its temperature. One set of transits announced the presence of a planet close to the star called TOI 700 b. Another set revealed a second planet, named TOI 700 c, a little farther out. The deeper, shorter transit means the planet is larger than the first and the plane of its orbit is slightly tipped, say experts. A final set of transits showed TOI 700 d, orbiting even farther out. 

TESS observed this system for nearly 11 months and saw each planet transit multiple times. Scientists determined that the inner and outer planets are almost Earth-sized and maybe rocky. The middle world is more than twice as large and most likely made of gas. All three may be tidally locked, rotating once each orbit, so the same side always faces the star. But most importantly, TOI 700 d is within the star's habitable zone, says the team.

“The innermost planet, called TOI 700 b, is almost exactly Earth-size, is probably rocky and completes an orbit every 10 days. The middle planet, TOI 700 c, is 2.6 times larger than Earth — between the sizes of Earth and Neptune — orbits every 16 days and is likely a gas-dominated world,” says the research team. 

They say: “TOI 700 d, the outermost known planet in the system and the only one in the habitable zone, measures 20% larger than Earth, orbits every 37 days and receives from its star 86% of the energy that the Sun provides to Earth. All of the planets are thought to be tidally locked to their star, which means they rotate once per orbit so that one side is constantly bathed in daylight.”

Scientists wanted independent confirmation of TOI 700 d. Accordingly, they monitored its star with NASA's Spitzer Space Telescope. Spitzer saw a clear transit from the outer planet, affirming its existence, and improving scientists' certainty of the planet size. The Spitzer data sharpened their measurements of its orbital period by 56% and its size by 38%. It also ruled out other possible astrophysical causes of the transit signal, such as the presence of a smaller, dimmer companion star in the system.

"Given the impact of this discovery — that it is TESS's first habitable-zone Earth-sized planet — we 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,” says Joseph Rodriguez, an astronomer at the Center for Astrophysics, Harvard & Smithsonian in Cambridge, Massachusetts. 

Because TOI 700 is bright and nearby and show no signs of stellar fares, the planets are good candidates for precise mass measurements by ground-based telescopes. These measurements could confirm scientists' estimates that the inner and outer planets are rocky and the middle planet is made of gas.

The three planets of the TOI 700 system orbit a small, cool M dwarf star. (NASA's Goddard Space Flight Center)

Future missions, say researchers, may also tell us if the planets have atmospheres. However, scientists need to know what kind of signals to look for. Accordingly, scientists at NASA"s Goddard Space Flight Center in Greenbelt, Maryland, created models of the planet to explore its potential conditions. 

One version is a water-covered world with an atmosphere similar to early Mars, but denser. Another looks like a completely dry version of today's Earth. Both models have vastly different surface temperatures. Light passing through their atmospheres creates distinct signals because different molecules are present. By simulating these data now, scientists can make predictions for real future observations, and narrow the range of TOI 700 d's possible conditions, say experts.

"Someday, when we have real spectra from TOI 700 d, we can backtrack, match them to the closest simulated spectrum, and then match that to a model. It's exciting because no matter what we find out about the planet, it's going to look completely different from what we have here on Earth,” says Gabrielle Engelmann-Suissa, a Universities Space Research Association visiting research assistant at Goddard.

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