The Transiting Exoplanet Survey Satellite (TESS) of National Aeronautics and Space Administration (NASA) has discovered its first Earth-size planet in the habitable zone of its star.
The planet has been found in the range of distances where the conditions are favourable for the presence of liquid water on its surface.
Scientists confirmed the find, called TOI 700 d, by using NASA's Spitzer Space Telescope and have modelled the potential environments of the planet for helping in informing future observations.
The planet TOI 700 d is one of the very few planets that are the size of Earth discovered in a habitable zone of a star. Other planets are the ones in the TRAPPIST-1 system and in the other worlds that have been 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," said Paul Hertz, director of the astrophysics division at NASA.
TESS functions by monitoring large swaths of the sky which are called sectors, for 27 days at a time. This long stare allows the satellite to track changes in stellar brightness caused by an orbiting planet crossing in front of its star from our perspective, an event called a transit.
TOI 700 is a small, cool M dwarf star located just over 100 light-years away in the southern constellation Dorado. It's roughly 40% of the Sun's mass and size and about half its surface temperature. 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.
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The star was originally misclassified in the TESS database as being more similar to our Sun, which meant the planets appeared larger and hotter than they really are. Several researchers, including Alton Spencer, a high school student working with members of the TESS team, identified the error.
"When we corrected the star's parameters, the sizes of its planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone. Additionally, in 11 months of data we saw no flares from the star, which improves the chances TOI 700 d is habitable and makes it easier to model its atmospheric and surface conditions," said Emily Gilbert, a researcher.
TOI 700 b, the innermost planet, is very close to being Earth-size. It is rocky and completes an orbit every 10 days. The middle planet which is TOI 700 c, is 2.6 times larger than Earth and is between the sizes of Earth and Neptune. It orbits every 16 days and is likely a gas-dominated world.
TOI 700 d is the outermost planet in the system and the only one in the habitable zone. It is 20 percent larger than Earth and orbits every 37 days. It receives 86 percent of the energy that Sun provides to Earth from its star. Every planet is thought to be tidally locked to their star, which means that they rotate only once per orbit and hence, one side of it is always bathed in daylight.
"Given the impact of this discovery -- that it is TESS's first habitable-zone Earth-sized planet -- we really 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," said lead researcher Joseph Rodriguez.
The data that scientists collected from the Spitzer gave them more confidence that TOI 700 d is a real planet and helped them sharpen their measurements of its orbital period by 56 percent and its size by 38 percent. It also ruled out other possible astrophysical causes of the transit signal, like that of the presence of a smaller, dimmer companion star in the system.
The researchers also used follow-up observations from a 1-meter ground-based telescope present in the global Las Cumbres Observatory network to improve scientists' confidence in the orbital period and size of TOI 700 c by 30 percent and 36 percent, respectively.
Since, TOI 700 is bright, near, and does not show signs of stellar flares, the system is a prime candidate for precise mass measurements by current ground-based observatories. These measurements could confirm the estimate of the scientists that the middle planet is made of gas and the inner and outer planets are rocky.
Future missions may be able to identify whether the planets have atmospheres and, if so, even determine their compositions.
While the exact conditions on TOI 700 d are unknown, scientists can use current information, like the planet's size and the type of star it orbits, to generate computer models and make predictions. Researchers at NASA's Goddard Space Flight Center in Greenbelt, Maryland, modeled 20 potential environments of TOI 700 d to gauge if any version would result in surface temperatures and pressures suitable for habitability.
Their 3D climate models examined a variety of surface types and atmospheric compositions typically associated with what scientists regard to be potentially habitable worlds. Since TOI 700 d is tidally locked to its star, the cloud formations of the planet and its wind patterns might be different from Earth's.
One simulation included an ocean-covered TOI 700 d with a dense, carbon-dioxide-dominated atmosphere similar to what scientists suspect surrounded Mars when it was young. The model atmosphere contains a deep layer of clouds on the star-facing side. Another model depicts TOI 700 d as a cloudless, all-land version of modern Earth, where winds flow away from the night side of the planet and converge on the point directly facing the star.
When the starlight passes through the atmosphere of a planet, it interacts with gaseous molecules like nitrogen and carbon dioxide and produces signals known as spectral lines. The researchers simulated spectra for the 20 modeled versions of TOI 700 d.
"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," said Gabrielle Engelmann-Suissa.