Credits: NASA, ESA, and J. Debes (STScI)
Searching for planets around other stars is a tricky
business. They're so small and faint that it's hard to spot them. But a
possible planet in a nearby stellar system may be betraying its presence
in a unique way: by a shadow that is sweeping across the face of a vast
pancake-shaped gas-and-dust disk surrounding a young star.
The planet itself is not casting the shadow. But it is doing some
heavy lifting by gravitationally pulling on material near the star and
warping the inner part of the disk. The twisted, misaligned inner disk
is casting its shadow across the surface of the outer disk.
A team of astronomers led by John Debes of the Space Telescope
Science Institute in Baltimore, Maryland, say this scenario is the most
plausible explanation for the shadow they spotted in the stellar system
TW Hydrae, located 192 light-years away in the constellation Hydra, also
known as the Female Water Snake. The star is roughly 8 million years
old and slightly less massive than our sun. The researchers uncovered
the phenomenon while analyzing 18 years' worth of archival observations
taken by NASA's Hubble Space Telescope.
"This is the very first disk where we have so many images over such a
long period of time, therefore allowing us to see this interesting
effect," Debes said. "That gives us hope that this shadow phenomenon may
be fairly common in young stellar systems."
Debes will present his team's results Jan. 7 at the winter meeting of the American Astronomical Society in Grapevine, Texas.
Debes' first clue to the phenomenon was a brightness in the disk that
changed with position. Astronomers using Hubble's Space Telescope
Imaging Spectrograph (STIS) first noted this brightness asymmetry in
2005. But they had only one set of observations, and could not make a
definitive determination about the nature of the mystery feature.
Searching the archive, Debes' team put together six images from
several different epochs. The observations were made by STIS and by
Hubble's Near Infrared Camera and Multi-Object Spectrometer (NICMOS).
STIS is equipped with a coronagraph that blocks starlight to within
about 1 billion miles from the star, allowing Hubble to look as close to
the star as Saturn is to our sun. Over time, the structure appeared to
move in counterclockwise fashion around the disk, until, in 2016, it was
in the same position as it was in images taken in 2000.
This 16-year period puzzled the researchers. They originally thought
the feature was part of the disk, but the short period meant that the
feature was moving way too fast to be physically in the disk.
Under the
laws of gravity, disks rotate at glacial speeds. The outermost parts of
the TW Hydrae disk would take centuries to complete one rotation.
"The fact that I saw the same motion over 10 billion miles from the
star was pretty significant, and told me that I was seeing something
that was imprinted on the outer disk rather than something that was
happening directly in the disk itself," Debes said. "The best
explanation is that the feature is a shadow moving across the surface of
the disk."
The research team concluded that whatever was making the shadow must
be deep inside the 41-billion-mile-wide disk, so close to the star it
cannot be imaged by Hubble or any other present-day telescope. The most
likely way to create a shadow is to have an inner disk that is tilted
relative to the outer disk. In fact, submillimeter observations of TW
Hydrae by the Atacama Large Millimeter Array (ALMA) in Chile suggested a
possible warp in the inner disk.
But what causes disks to warp? "The most plausible scenario is the
gravitational influence of an unseen planet, which is pulling material
out of the plane of the disk and twisting the inner disk," Debes
explained. "The misaligned disk is inside the planet's orbit."
Given the relatively short 16-year period of the clocklike moving
shadow, the planet is estimated to be about 100 million miles from the
star — about as close as Earth is from the sun. The planet would be
roughly the size of Jupiter to have enough gravity to pull the material
up out of the plane of the main disk. The planet's gravitational pull
causes the disk to wobble, or precess, around the star, giving the
shadow its 16-year rotational period.
Recent observations of TW Hydrae by ALMA in Chile add credence to the
presence of a planet. ALMA revealed a gap in the disk roughly 9 million
miles from TW Hydrae. A gap is significant, because it could be the
signature of an unseen planet clearing away a path in the disk.
This new Hubble study offers a unique way to look for planets hiding
in the inner part of the disk and probe what is happening very close to
the star, which is not reachable in direct imaging by current
telescopes. "What is surprising is that we can learn something about an
unseen part of the disk by studying the disk's outer region and by
measuring the motion, location, and behavior of a shadow," Debes said.
"This study shows us that even these large disks, whose inner regions
are unobservable, are still dynamic, or changing in detectable ways
which we didn't imagine."
The Hubble Space Telescope is a project of international cooperation
between NASA and the European Space Agency. NASA's Goddard Space Flight
Center in Greenbelt, Maryland, manages the telescope. The Space
Telescope Science Institute (STScI) in Baltimore, Maryland, conducts
Hubble science operations. STScI is operated for NASA by the Association
of Universities for Research in Astronomy in Washington, D.C.
Contact
Felicia Chou
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
John Debes
Space Telescope Science Institute, Baltimore, Maryland
410-338-4782
debes@stsci.edu
NASA Headquarters, Washington, D.C.
202-358-0257
felicia.chou@nasa.gov
Donna Weaver / Ray Villard
Space Telescope Science Institute, Baltimore, Maryland
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
John Debes
Space Telescope Science Institute, Baltimore, Maryland
410-338-4782
debes@stsci.edu
Source: HubbleSite