Astronomers have discovered thousands of exoplanets — planets beyond our solar
system — but few have been directly imaged, because they are extremely
difficult to see with existing telescopes. A University of Hawaiʻi Institute
for Astronomy (IfA) graduate student has beaten the odds and discovered a
directly imaged exoplanet, and it’s the closest one to Earth ever found, at a
distance of only 35 light years.
Using the COol Companions ON Ultrawide orbiTS (COCONUTS) survey, IfA graduate
student Zhoujian Zhang and a team of astronomers, Michael Liu and Zach Claytor
(IfA), William Best (University of Texas at Austin), Trent Dupuy (University
of Edinburgh) and Robert Siverd (Gemini Observatory/National Optical-Infrared
Astronomy Research Laboratory) identified a planet about six times the mass of
Jupiter. The team’s research, published in The Astrophysical Journal Letters,
led to the discovery of the low-temperature gas-giant planet orbiting a
low-mass red dwarf star, about 6,000 times farther than the Earth orbits the
Sun. They dubbed the new planetary system COCONUTS-2, and the new planet
COCONUTS-2b.
“With a massive planet on a super-wide-separation orbit, and with a very cool
central star, COCONUTS-2 represents a very different planetary system than our
own solar system,” Zhang explained. The COCONUTS survey has been the focus of
his recently-completed PhD thesis, aiming to find wide-separation companions
around stars of all different types close to Earth.
Trapped heat helps detect planet
COCONUTS-2b is the second-coldest imaged exoplanet found to date, with a
temperature of just 320 degrees Fahrenheit, which is slightly cooler than
most ovens use to bake cookies. The planet can be directly imaged thanks to
emitted light produced by residual heat trapped since the planet’s
formation. Still, the energy output of the planet is more than a million
times weaker than the Sun’s, so the planet can only be detected using
lower-energy infrared light.
“Directly detecting and studying the light from gas-giant planets around
other stars is ordinarily very difficult, since the planets we find usually
have small-separation orbits and thus are buried in the glare of their host
star’s light,” said Liu, Zhang’s thesis advisor. “With its huge orbital
separation, COCONUTS-2b will be a great laboratory for studying the
atmosphere and composition of a young gas-giant planet.”
The planet was first detected in 2011 by the Wide-field Infrared Survey
Explorer satellite, but it was believed to be a free-floating object, not
orbiting a star. Zhang and his collaborators discovered that it is in fact
gravitationally bound to a low-mass star, COCONUTS-2A, which is about
one-third the mass of the Sun, and about 10 times younger.
Darkness prevails
Due to its wide-separation orbit and cool host star, COCONUTS-2b’s skies
would look dramatically different to an observer there compared to the skies
on Earth. Nighttime and daytime would look basically the same, with the host
star appearing as a bright red star in the dark sky.
Zhang’s discovery has fueled his desire to continue to explore exoplanets,
brown dwarfs, and stars. The aspiring astronomer graduated from IfA this
summer and will begin his postdoctoral research in fall 2021, with IfA
alumnus Brendan Bowler, an astronomy professor at the University of Texas at
Austin.
Reference:
“The Second Discovery from the COCONUTS Program: A Cold Wide-orbit Exoplanet
around a Young Field M Dwarf at 10.9 pc” by Zhoujian Zhang, Michael C. Liu,
Zachary R. Claytor, William M. J. Best, Trent J. Dupuy and Robert J. Siverd,
28 July 2021, The Astrophysical Journal Letters.
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Space & Astrophysics