Weird 'Sub-Neptune' Exoplanet Discovered by NASA Space Telescope

Cristina Cross
January 9, 2019

It orbits a red dwarf star around 40 million light years away, and scientists think it holds giant oceans of magma.

The National Aeronautics and Space Administration (Nasa) which launched Transiting Exoplanet Survey Satellite (Tess) for searching exoplanets in April, 2018 has discovered a third small planet outside our solar system. That is fast by Earth standards, but the other two planets include Pi Mensae b with a 6.3-day orbit and LHS 3844b that orbits its star at a blistering pace of once every 11 hours.

A year ago at the American Astronomical Society meeting, it was announced that citizen scientists helped discover five planets between the size of Earth and Neptune around star K2-138, the first multiplanet system found through crowdsourcing.

"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. "But here we were lucky, and caught this one, and can now study it in more detail", Dragomir said.

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. It is also a whopping 23 times as massive as Earth. 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.

Johanna Teske, a Hubble fellow and co-author of the report, said: "I'm very interested to know whether [it] has an Earth-like density to match its Earth-like radius - this will contribute to our understanding whether Earth-sized planets have diverse compositions or are all roughly similar to Earth".

The researchers also detected hints of another, smaller planet in the system, a planet that would have an orbital period of 7.8 days. During the meeting, Chelsea Huang, an MIT researcher working on the TESS datasets, stated that TESS has confirmed the existence of eight new exoplanets, and that "there are probably 20 or 30" other potential discoveries "almost ready to be published".

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The new planet is named HD 21749b.

Tess monitors sections of the sky and waits for momentary dips in the light of about 200,000 nearby stars - a sign that a planet has passed in front of that star.

This shifting focus makes it tough for TESS to find planets that lie far from their host stars and therefore take a long time to complete one orbit.

TESS will continue to sweep the southern hemisphere until mid-2019, at which point it will turn its cameras to the Northern Hemisphere and start another observation phase. This is relatively cool considering its proximity to its star. "But we had this one transit, and knew something was there".

The planet hunter's latest discovery-a small planet outside our Solar System-was announced at this week's American Astronomical Society meeting in Seattle. They determined that they should be able to find the signal again, in TESS's "sector 3" data - which they succeeded in doing. For what it's worth, several TESS finds have been the subject of pre-print research papers.

In total, Kepler has found around 5,000 unconfirmed "candidate" exoplanets, with a further 2,500 "confirmed" exoplanets that scientists have since shown to be real. "So it's going really well, and TESS is already helping us to learn about the diversity of these small planets".

Huang said several more planets have had their status confirmed, but she didn't provide specifics.

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