NASA Missions Spy First Possible Planet Hugging a Stellar Cinder
The violent events leading up to the death of a star would likely drive away any planets. The newly discovered Jupiter-size object may have arrived long after the star died.
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, itejects 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 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, however, 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 was 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 were 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 discernable
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 was located 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 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
|
