New planet discovered by Nasa's TESS probe is thrice Earth's size

Rodiano Bonacci
Gennaio 9, 2019

NASA's planet-hunting telescope is making landmark discoveries at an astonishing pace and has now confirmed discovery of a third new planet and a handful of exploding stars in our "cosmic backyard".

It has since been replaced by NASA's new space telescope, the Transiting Exoplanet Survey Satellite (TESS).

The nearby exoplanet, HD 21749b, orbits a bright neighboring star in the Reticulum constellation, with a 36-day orbit and a surface temperature of 300 degrees Fahrenheit. HD 21749b journeys around its star in a relatively leisurely 36 days, compared to the two other planets - Pi Mensae b, a "super-Earth" with a 6.3-day orbit, and LHS 3844b, a rocky world that speeds around its star in just 11 hours. The team, however, needed at least three transits to claim the discovery of a candidate planet and they did not find a third signal in the observations they reviewed.

"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", she added. "But here we were lucky, and can now study this one in more detail".

About three times the size of Earth (and a whopping 23 times as massive), HD 21749b is categorized as a "sub-Neptune".

"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 Feinstein, who presented the discovery at the 233rd meeting of the American Astronomical Society in Seattle on Monday, Jan. 7. But the gas that makes it up it is probably more dense than that of Uranus or Neptune.

"We think this planet wouldn't be as gaseous as Neptune or Uranus, which are mostly hydrogen and really puffy", Dragomir says. "The planet likely has a density of water, or a thick atmosphere", Dragomir said.

They searched Kepler data for evidence of transits, the regular dimming of a star when an orbiting planet moves across the star's face.

The stellar system has two dim, cool M-type stars about 5.1 billion miles apart, which is about six times the distance between Saturn and the sun. The satellite will spend the first year surveying the sky in the Southern Hemisphere, before swiveling around to take in the Northern Hemisphere sky.

A different group of astronomers had studied the HD 21749 system a decade ago using HARPS, which finds planets by noticing the tiny wobbles that their gravitational tugs induce in their host stars. "But we had this one transit, and knew something was there".

As it turned out, though, the team wasn't actually analyzing all of the data.

"Because there was an interruption in data around that time, we initially didn't see a second transit, and were pretty disappointed", Dragomir recalls.

The newfound planet K2-288Bb, illustrated here, is slightly smaller than Neptune. "But we re-extracted the data and zoomed in to look more carefully, and found what looked like the end of a transit". Those researchers had also detected a signal, but they couldn't conclusively attribute it to a planet, Dragomir said. The spacecraft ran out of fuel in October 2018, nine years after it was launched into space to hunt for Earth-like planets. "So it's going really well, and TESS is already helping us to learn about the diversity of these small planets".

"This is only the ninth system discovered containing six or more planets", he said.

TESS is a NASA Astrophysics Explorer mission led and operated by MIT in Cambridge, Massachusetts, and managed by Goddard.

An illustration of NASA's Kepler space telescope. More than a dozen universities, research institutes, and observatories worldwide are participants in the mission.

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