It orbits a red dwarf star around 40 million light years away, and scientists think it holds giant oceans of magma.
The other two planets it has discovered are Pi Mensae c, a super-Earth that zips around its star in 6.3 days, and LHS 3844b, a rocky planet that flies around its planet in a whopping 11-hour orbit.
An exoplanet three times the size of Earth has been discovered relatively close to our solar system.
"It's a very exciting discovery due to how it was found, its temperate orbit and because planets of this size seem to be relatively uncommon", said Adina Feinstein, a University of Chicago graduate student in astrophysics and lead author of a paper describing the new planet that was accepted for publication by The Astronomical Journal. "We know a lot about atmospheres of hot planets, but because it's very hard to find small planets that orbit farther from their stars and are therefore cooler, we haven't been able to learn much about these smaller, cooler planets".
Calling the planet small is a bit misleading, the team says that HD 21749b is about three times the size of Earth placing it into the sub-Neptune category. In comparison, Neptune is nearly four times as wide as Earth but only 17 times as massive. But it is unlikely that the planet is rocky and therefore habitable; it's more likely made of gas, of a kind that is much more dense than the atmospheres of either Neptune or Uranus.
Researcher Diana Dragomir said the planet would not look anything like Earth.
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The researchers also detected hints of another, smaller planet in the system, a planet that would have an orbital period of 7.8 days.
Citizen scientists have discovered a potentially habitable exoplanet roughly twice the size of Earth, NASA's Jet Propulsion Laboratory said Monday.
TESS will look for exoplanets using the transit method, observing slight dips in the brightness of stars as planets pass in front of them. "TESS found as many in its first month".
Even though the Kepler spacecraft ceased operations months ago, after almost a decade in service, its legacy continues: Today, researchers announced that they have found a planet roughly twice as big as Earth, located within what could be its parent star's habitable zone. "We already have six in one month", Fausnaugh said.
Kepler found a planet that orbits two stars, known as a binary star system, in 2011. It was unclear whether this signal was caused by a planet or variations in the host star's activity, so Dragomir and her colleagues analyzed observations taken by another instrument, the High Accuracy Radial velocity Planet Searcher (HARPS), a spectrograph installed on a telescope at the European Southern Observatory's La Silla Observatory in Chile.
The planet is in the K2-288 system, which contains a pair of dim, cool M-type stars that are 5.1 billion miles apart, about six times the distance between Saturn and the sun. "So it's going really well, and TESS is already helping us to learn about the diversity of these small planets".
"We've confirmed three planets so far, and there are so many more that are just waiting for telescope and people time to be confirmed", Dragomir says.
In 2014 the telescope made one of its biggest discoveries when it spotted exoplanet Kepler-452b, dubbed "Earth 2.0".