Credit: NASA, ESA, J. Debes (STScI),
H. Jang-Condell (University of Wyoming), A. Weinberger (Carnegie
Institution of Washington), A. Roberge (Goddard Space Flight Center), G.
Schneider (University of Arizona/Steward Observatory), and A. Feild (STScI/AURA)
Nearly 900 extrasolar planets have been confirmed to date, but now
for the first time astronomers think they are seeing compelling
evidence for a planet under construction in an unlikely place, at a
great distance from its diminutive red dwarf star.
The keen vision of NASA's Hubble Space Telescope has detected a
mysterious gap in a vast protoplanetary disk of gas and dust swirling
around the nearby star TW Hydrae, located 176 light-years away in the
constellation Hydra (the Sea Serpent). The gap's presence is best
explained as due to the effects of a growing, unseen planet that is
gravitationally sweeping up material and carving out a lane in the disk,
like a snow plow.
Researchers, led by John Debes of the Space Telescope Science
Institute in Baltimore, Md., found the gap about 7.5 billion miles from
the red dwarf star. If the putative planet orbited in our solar
system, it would be roughly twice Pluto's distance from the Sun.
The suspected planet's wide orbit means that it is moving slowly
around its host star. Finding the suspected planet in this orbit
challenges current planet formation theories. The conventional
planet-making recipe proposes that planets form over tens of millions of
years from the slow but persistent buildup of dust, rocks, and gas as a
budding planet picks up material from the surrounding disk. TW Hydrae,
however, is only 8 million years old. There has not been enough time
for a planet to grow through the slow accumulation of smaller debris.
In fact, a planet at 7.5 billion miles from its star would take more
than 200 times longer to form than Jupiter did at its distance from the
Sun because of its much slower orbital speed and a deficiency of
material in the disk.
An alternative planet-formation theory suggests that a piece of the
disk becomes gravitationally unstable and collapses on itself. In this
scenario, a planet could form more quickly, in just a few thousand
years.
"If we can actually confirm that there's a planet there, we can
connect its characteristics to measurements of the gap properties,"
Debes says. "That might add to planet formation theories as to how you
can actually form a planet very far out. There's definitely a gap
structure. We think it's probably a planet given the fact that the gap
is sharp and circular."
What complicates the story is that the red dwarf star is only 55
percent the mass of our Sun. "It's so intriguing to see a system like
this," Debes says. "This is the lowest-mass star for which we've
observed a gap so far out."
The disk also lacks large dust grains in its outer regions.
Observations from ALMA (the Atacama Large Millimeter Array) show that
millimeter-sized (tenths-of-an-inch-sized) dust, roughly the size of a
grain of sand, cuts off sharply at about 5.5 billion miles from the
star, just short of the gap. The disk is 41 billion miles across.
"Typically, you need pebbles before you can have a planet. So, if
there is a planet and there is no dust larger than a grain of sand
farther out, that would be a huge challenge to traditional
planet-formation models," Debes says.
The Hubble observations reveal that the gap, which is 1.9 billion
miles wide, is not completely cleared out. The team suggests that if a
planet exists, it is in the process of forming and not very massive.
Based on the evidence, team member Hannah Jang-Condell at the
University of Wyoming in Laramie estimates that the putative planet is 6
to 28 times more massive than Earth. Within this range lies a class of
planets called super-Earths and ice giants. Such a small planet mass
is also a challenge to direct-collapse planet-formation theories, which
predict that clumps of material one to two times more massive than
Jupiter can collapse to form a planet.
TW Hydrae has been a popular target with astronomers. The system is
one of the closest examples of a face-on disk, giving astronomers an
overhead view of the star's environment. Debes's team used Hubble's
Near Infrared Camera and Multi-Object Spectrometer (NICMOS) to observe
the star in near-infrared light. The team then re-analyzed archival
Hubble data, using more NICMOS images as well as optical and
spectroscopic observations from the Space Telescope Imaging
Spectrograph (STIS). Armed with these observations, they composed the
most comprehensive view of the system in scattered light over many
wavelengths.
When Debes accounted for the rate at which the disk dims from
reflected starlight, the gap was highlighted. It was a feature that two
previous Hubble studies had suspected but could not definitively
confirm. These earlier observations noted an uneven brightness in the
disk but did not identify it as a gap.
"When I first saw the gap structure, it just popped out like that,"
Debes says. "The fact that we see the gap at every wavelength tells you
that it's a structural feature rather than an instrumental artifact or
a feature of how the dust scatters light.
The team plans to use ALMA and NASA's upcoming James Webb Space
Telescope, an infrared observatory set to launch in 2018, to study the
system in more detail.
The team's paper will appear online on June 14 in The Astrophysical Journal.
CONTACT
Donna Weaver / Ray VillardSpace Telescope Science Institute, Baltimore, Md.
410-338-4493 / 410-338-4514
dweaver@stsci.edu / villard@stsci.edu
John Debes
Space Telescope Science Institute, Baltimore, Md.
410-338-4782
debes@stsci.edu
