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A NASA mission has discovered an Earth-sized exoplanet orbiting a small star about 100 light-years away.
The planet, named TOI 700 e, is likely to be rocky and 95% the size of our world. The celestial body is the fourth planet detected orbiting the small, cool dwarf star M TOI 700. All the exoplanets they were found by NASA’s Transiting Exoplanet Survey Satellite, or TESS mission.
Another planet in the system, discovered in 2020 and named TOI 700 d, is also the size of Earth. Both exoplanets exist in their star’s habitable zone, or just the right distance from the star where liquid water could potentially exist on their surfaces. The potential for liquid water suggests that the planets themselves could be, or could once have been, habitable for life.
The discovery of the fourth planet was announced Tuesday at the 241st meeting of the American Astronomical Society in Seattle, and a study on the exoplanet has been accepted for publication by The Astrophysical Journal Letters.
“This is one of the few systems with multiple small planets in habitable zones that we know of,” lead study author Emily Gilbert, a postdoctoral fellow at NASA’s Jet Propulsion Laboratory in Pasadena, California, said in a statement.
“That makes the TOI 700 system an exciting prospect for further monitoring. Planet e is about 10% smaller than planet d, so the system also shows how additional TESS observations help us find ever smaller worlds.”
Small, cool M dwarf stars like TOI 700 are common in the universe, and in recent years many have been found to harbor exoplanets, such as the The TRAPPIST-1 system and its seven exoplanets which will be observed by the James Webb Space Telescope.
The closest to the star is TOI 700 b, which is 90% the size of Earth and completes a rapid orbit around the star every 10 Earth days. Then there’s TOI 700 c, which is 2.5 times bigger than our planet and completes one orbit around the star every 16 days. Both planets are likely tidally locked, meaning they always face the same side of the star, just as the same side of the moon always faces Earth.
The two exoplanets in the star’s habitable zone, the dye planets, have longer orbits of 37 and 28 days, respectively, because they are slightly further from the star. The newly announced planet e is actually located between planets c and d.
The TESS mission, launched in 2018, monitors large swathes of the night sky for 27 days straight, observing the brightest stars and tracking their changes in brightness. These dips in luminosity indicate the orbiting planets as they pass in front of their stars, called transits. The mission began observing the southern sky in 2018 and then headed for the northern sky. In 2020, the mission refocused on the southern sky for additional observations, revealing the fourth planet in the TOI 700 system.
“If the star were a little closer or the planet a little bigger, we could have detected TOI 700 e in the first year of TESS data,” said study co-author Ben Hord, a doctoral student at the University of Maryland, College Park, and a graduate researcher at NASA’s Goddard Space Flight Center in Greenbelt, Maryland, in a statement. “But the signal was so weak that we needed an additional year of transit observations to identify it.”
While the researchers use other space-based and ground-based observatories to make follow-up observations of the intriguing planetary system, more data is coming in from TESS.
“TESS has just completed its second year of northern sky observations,” said Allison Youngblood, a research astrophysicist and associate TESS project scientist at Goddard. “We look forward to the other exciting discoveries hidden in the mission’s treasure trove of data.”
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