WASHINGTON, Sept. 16, 2020 /PRNewswire/ -- An international
team of astronomers using NASA's Transiting Exoplanet Survey
Satellite (TESS) and retired Spitzer Space Telescope has
reported what may be the first intact planet found closely orbiting
a white dwarf, the dense leftover of a Sun-like star, only 40%
larger than Earth.
The Jupiter-size object, called WD 1856
b, is about seven times larger than the white dwarf, named
WD 1856+534. It circles this stellar cinder every 34 hours, more
than 60 times faster than Mercury orbits our Sun.
"WD 1856 b somehow got very close
to its white dwarf and managed to stay in one piece," said
Andrew Vanderburg, an assistant
professor of astronomy at the University of
Wisconsin-Madison. "The white dwarf creation process
destroys nearby planets, and anything that later gets too close is
usually torn apart by the star's immense gravity. We still have
many questions about how WD 1856 b
arrived at its current location without meeting one of those
fates."
A paper about the system, led by Vanderburg and including
several NASA co-authors, appears in the Sept. 17 issue of Nature and is now available
online.
TESS monitors large swaths of the sky, called sectors, for
nearly a month at a time. This long gaze allows the satellite to
find exoplanets, or worlds beyond our solar system, by capturing
changes in stellar brightness caused when a planet crosses in front
of, or transits, its star.
The satellite spotted WD 1856 b
about 80 light-years away in the northern constellation Draco. It
orbits a cool, quiet white dwarf that is roughly 11,000 miles
(18,000 kilometers) across, may be up to 10 billion years old, and
is a distant member of a triple star system.
When a Sun-like star runs out of fuel, it swells up to hundreds
to thousands of times its original size, forming a cooler red
giant star. Eventually, it ejects its outer layers of
gas, losing up to 80% of its mass. The remaining hot core becomes a
white dwarf. Any nearby objects are typically engulfed and
incinerated during this process, which in this system would have
included WD 1856 b in its current
orbit. Vanderburg and his colleagues estimate the possible planet
must have originated at least 50 times farther away from its
present location.
"We've known for a long time that after white dwarfs are born,
distant small objects such as asteroids and comets can
scatter inward towards these stars. They're usually pulled
apart by a white dwarf's strong gravity and turn into a debris
disk," said co-author Siyi Xu, an
assistant astronomer at the international Gemini Observatory in
Hilo, Hawaii, which is a program
of the National Science Foundation's NOIRLab. "That's why I was so
excited when Andrew told me about this system. We've seen
hints that planets could scatter inward, too, but this appears
to be the first time we've seen a planet that made the whole
journey intact."
The team suggests several scenarios that could have nudged WD
1856 b onto an elliptical path around
the white dwarf. This trajectory would have become more circular
over time as the star's gravity stretched the object, creating
enormous tides that dissipated its orbital energy.
"The most likely case involves several other Jupiter-size bodies
close to WD 1856 b's original orbit," said co-author Juliette Becker, a 51 Pegasi b Fellow in
planetary science at Caltech (California
Institute of Technology) in Pasadena. "The gravitational
influence of objects that big could easily allow for the
instability you'd need to knock a planet inward. But at this point,
we still have more theories than data points."
Other possible scenarios involve the gradual gravitational tug
of the two other stars in the system, red dwarfs G229-20 A and B,
over billions of years and a flyby from a rogue star perturbing the
system. Vanderburg's team thinks these and other explanations are
less likely because they require finely tuned conditions to achieve
the same effects as the potential giant companion planets.
Jupiter-size objects can occupy a huge range of masses, from
planets only a few times more massive than Earth to low-mass
stars thousands of times Earth's mass. Others are brown dwarfs,
which straddle the line between planet and star. Usually scientists
turn to radial velocity observations to measure an object's
mass, which can hint at its composition and nature. This method
works by studying how an orbiting object tugs on its star and
alters the color of its light. But in this case, the white dwarf is
so old that its light has become both too faint and too featureless
for scientists to detect noticeable changes.
Instead, the team observed the system in the infrared using
Spitzer, just a few months before the telescope was decommissioned.
If WD 1856 b were a brown dwarf or
low-mass star, it would emit its own infrared glow. This means
Spitzer would record a brighter transit than it would if the object
was a planet, which would block rather than emit light. When the
researchers compared the Spitzer data to visible light transit
observations taken with the Gran Telescopio Canarias in
Spain's Canary Islands, they saw
no discernible difference. That, combined with the age of the star
and other information about the system, led them to conclude that
WD 1856 b is most likely a planet no
more than 14 times Jupiter's size. Future research and observations
may be able to confirm this conclusion.
Finding a possible world closely orbiting a white dwarf prompted
co-author Lisa Kaltenegger,
Vanderburg, and others to consider the implications for studying
atmospheres of small rocky worlds in similar situations. For
example, suppose that an Earth-size planet were located within the
range of orbital distances around WD 1856 where water could exist
on its surface. Using simulated observations, the researchers show
that NASA's upcoming James Webb Space Telescope could detect
water and carbon dioxide on the hypothetical world by observing
just five transits.
The results of these calculations, led by Kaltenegger and
Ryan MacDonald, both at Cornell
University in Ithaca, New
York, have been published in The Astrophysical Journal
Letters and are available online.
"Even more impressively, Webb
could detect gas combinations potentially indicating biological
activity on such a world in as few as 25 transits," said
Kaltenegger, the director of Cornell's
Carl Sagan Institute. "WD 1856 b
suggests planets may survive white dwarfs' chaotic histories. In
the right conditions, those worlds could maintain conditions
favorable for life longer than the time scale predicted for Earth.
Now we can explore many new intriguing possibilities for worlds
orbiting these dead stellar cores."
There is currently no evidence suggesting there are other worlds
in the system, but it's possible additional planets exist and
haven't been detected yet. They could have orbits that exceed the
time TESS observes a sector or are tipped in a way such that
transits don't occur. The white dwarf is also so small that the
possibility of catching transits from planets farther out in the
system is very low.
TESS is a NASA Astrophysics Explorer mission led and operated by
MIT in Cambridge, Massachusetts, and managed by
NASA's Goddard Space Flight Center in Greenbelt, Maryland. 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 Jet Propulsion Laboratory (JPL) in Southern California managed the Spitzer
mission for the agency's Science Mission Directorate in
Washington. Spitzer science data
continue to be analyzed by the science community via the Spitzer
data archive, located at the Infrared Science Archive housed at the
Infrared Processing and Analysis Center (IPAC) at Caltech. Science
operations were conducted at the Spitzer Science Center at Caltech.
Spacecraft operations were based at Lockheed Martin Space in
Littleton, Colorado. Caltech
manages JPL for NASA.
For more information on TESS, visit:
https://www.nasa.gov/tess
For more information on Spitzer, visit:
https://www.nasa.gov/spitzer
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