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Weird ‘sub-Neptune’
exoplanet found by TESS
he Transiting Exoplanet Survey
Satellite (TESS) has discovered a
planet orbiting the star HD 21749,
which lies about 53 light years
from Earth in the faint constellation Reticulum,
scientists have announced.
Theworld,knownasHD21749b,isaweirdo-at
least by the standards of our own Solar System.
It's a ‘sub-Neptune’ about three-times bigger than
Earth, which means it's likely gaseous rather
thanrocky.ButHD21749bseemstobemadeof
denser stuff than the gaseous worlds we're used to,
because it's 23-times more massive than Earth.
“We think this planet wouldn't be as gaseous
as Neptune or Uranus, which are mostly hydrogen
and really puffy,” discovery team leader Diana
Dragomir of the Massachusetts Institute of
Technology's Kavli Institute for Astrophysics andSpaceResearchsaid.“Theplanetlikelyhasa
density of water, or a thick atmosphere.”
HD 21749b completes one orbit of its host star,
whichisnearlyasbrightasourSun,every36Earth
days. The exoplanet likely has an average surface
temperature around 150 degrees Celsius (
degrees Fahrenheit), discovery team members said.
The researchers also detected hints of another,
smaller planet in the system, a planet that would
haveanorbitalperiodof7.8days.Ifthisworldis
confirmed it will be the first roughly Earth-size
planet found by TESS.
TESS launched to Earth orbit in April 2018 on
a mission to find alien worlds circling stars
in our Solar System's neighbourhood. Like
NASA's recently deceased Kepler space telescope,
which is responsible for about 70 per cent of all
known exoplanet discoveries to date, TESS usesthe transit method. That is, it looks for tiny dips in
starlight caused when planets cross their host stars'
facesfromthesatellite'sperspective.
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. Indeed, HD 21749b is very far-f lung for
TESS; two other smallish worlds found by the
mission have orbital periods of 11 hours and 6.
days respectively.
“It's the coolest small planet that we know of
around a star this bright,” Dragomir said. “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. Here we were lucky and caught this one
and can now study it in more detail.”T
© NASA; ESA; Jingchuan Yu, Beijing Planetarium
TESS was
launched in
April 2018
to search for
exoplanetsIN COOPER TION WITINCOOPERATIONWITH
Words by Mike Wall“ HD 21749b seems to be made of
denser stuff than the gaseous
worlds we're used to”