Tour the GJ 357 system, located 31 light-years away
in the constellation Hydra. Astronomers confirming a planet candidate
identified by NASA’s Transiting Exoplanet Survey Satellite subsequently
found two additional worlds orbiting the star. The outermost planet, GJ
357 d, is especially intriguing to scientists because it receives as
much energy from its star as Mars does from the Sun. Credits: NASA’s Goddard Space Flight Center. Download this video and related multimedia in HD formats from NASA Goddard's Scientific Visualization Studio
A piping hot planet discovered by NASA’s Transiting Exoplanet Survey Satellite (TESS) has pointed the way to additional worlds orbiting the same star, one of which is located in the star’s habitable zone. If made of rock, this planet may be around twice Earth’s size.
The new worlds orbit a star named GJ 357, an M-type dwarf about
one-third the Sun’s mass and size and about 40% cooler that our star.
The system is located 31 light-years away in the constellation Hydra. In
February, TESS cameras caught the star dimming slightly every 3.9 days,
revealing the presence of a transiting
exoplanet — a world beyond our solar system — that passes across the
face of its star during every orbit and briefly dims the star’s light.
“In a way, these planets were hiding in measurements made at numerous
observatories over many years,” said Rafael Luque, a doctoral student
at the Institute of Astrophysics of the Canary Islands (IAC) on Tenerife who led the discovery team. “It took TESS to point us to an interesting star where we could uncover them.”
The transits TESS observed belong to GJ 357 b, a planet about 22%
larger than Earth. It orbits 11 times closer to its star than Mercury
does our Sun. This gives it an equilibrium temperature — calculated
without accounting for the additional warming effects of a possible
atmosphere — of around 490 degrees Fahrenheit (254 degrees Celsius).
“We describe GJ 357 b as a ‘hot Earth,’” explains co-author Enric
Pallé, an astrophysicist at the IAC and Luque’s doctoral supervisor.
“Although it cannot host life, it is noteworthy as the third-nearest
transiting exoplanet known to date and one of the best rocky planets we
have for measuring the composition of any atmosphere it may possess.”
But while researchers were looking at ground-based data to confirm
the existence of the hot Earth, they uncovered two additional worlds.
The farthest-known planet, named GJ 357 d, is especially intriguing.
This diagram shows the layout of the GJ 357 system.
Planet d orbits within the star’s so-called habitable zone, the orbital
region where liquid water can exist on a rocky planet’s surface. If it
has a dense atmosphere, which will take future studies to determine, GJ
357 d could be warm enough to permit the presence of liquid water.Credits: NASA's Goddard Space Flight Center/Chris Smith
“GJ 357 d is located within the outer edge of its star’s habitable
zone, where it receives about the same amount of stellar energy from its
star as Mars does from the Sun,” said co-author Diana Kossakowski at
the Max Planck Institute for Astronomy in Heidelberg, Germany. “If the
planet has a dense atmosphere, which will take future studies to
determine, it could trap enough heat to warm the planet and allow liquid
water on its surface.”
Without an atmosphere, it has an equilibrium temperature of -64 F
(-53 C), which would make the planet seem more glacial than habitable.
The planet weighs at least 6.1 times Earth’s mass, and orbits the star
every 55.7 days at a range about 20% of Earth’s distance from the Sun.
The planet’s size and composition are unknown, but a rocky world with
this mass would range from about one to two times Earth’s size.
Credits: NASA's Goddard Space Flight Center/Chris Smith
Even through TESS monitored the star for about a month, Luque’s team
predicts any transit would have occurred outside the TESS observing
window.
GJ 357 c, the middle planet, has a mass at least 3.4 times Earth’s,
orbits the star every 9.1 days at a distance a bit more than twice that
of the transiting planet, and has an equilibrium temperature around 260 F
(127 C). TESS did not observe transits from this planet, which suggests
its orbit is slightly tilted — perhaps by less than 1 degree — relative
to the hot Earth’s orbit, so it never passes across the star from our
perspective.
To confirm the presence of GJ 357 b and discover its neighbors, Luque
and his colleagues turned to existing ground-based measurements of the
star’s radial velocity,
or the speed of its motion along our line of sight. An orbiting planet
produces a gravitational tug on its star, which results in a small
reflex motion that astronomers can detect through tiny color changes in
the starlight. Astronomers have searched for planets around bright stars
using radial velocity data for decades, and they often make these
lengthy, precise observations publicly available for use by other
astronomers.
Luque’s team examined ground-based data stretching back to 1998 from
the European Southern Observatory and the Las Campanas Observatory in
Chile, the W.M. Keck Observatory in Hawaii, and the Calar Alto
Observatory in Spain, among many others.
A paper describing the findings was published on Wednesday, July 31, in the journal Astronomy & Astrophysics and is available online.
TESS is a NASA Astrophysics Explorer mission led and operated by MIT in
Cambridge, Massachusetts, and managed by NASA's Goddard Space Flight
Center. Additional partners include Northrop Grumman, based in Falls
Church, Virginia; NASA’s Ames Research Center in California’s Silicon
Valley; the Harvard-Smithsonian Center for Astrophysics in Cambridge,
Massachusetts; MIT’s Lincoln Laboratory; and the Space Telescope Science
Institute in Baltimore. More than a dozen universities, research
institutes and observatories worldwide are participants in the mission.
NASA’s Goddard Space Flight Center, Greenbelt, Md.
Editor: Rob Garner
Source: NASA/TESS
