NASA'S Hubble Uncovers Evidence of Farthest Planet Forming From Its Star

June 13, 2013

J.D. Harrington
Headquarters, Washington
202-358-5241
j.d.harrington@nasa.gov

Donna Weaver
Space Telescope Science Institute, Baltimore, Md.
410-338-4593
dweaver@stsci.edu


RELEASE: 13-182

NASA'S HUBBLE UNCOVERS EVIDENCE OF FARTHEST PLANET FORMING FROM ITS STAR

WASHINGTON -- Astronomers using NASA's Hubble Space Telescope have
found compelling evidence of a planet forming 7.5 billion miles away
from its star, a finding that may challenge current theories about
planet formation.

Of the almost 900 planets outside our solar system that have been
confirmed to date, this is the first to be found at such a great
distance from its star. The suspected planet is orbiting the
diminutive red dwarf TW Hydrae, a popular astronomy target located
176 light-years away from Earth in the constellation Hydra the Sea
Serpent.

Hubble's keen vision detected a mysterious gap in a vast
protoplanetary disk of gas and dust swirling around TW Hydrae. The
gap is 1.9 billion miles wide and the disk is 41 billion miles wide.
The gap's presence likely was caused by a growing, unseen planet that
is gravitationally sweeping up material and carving out a lane in the
disk, like a snow plow.

The planet is estimated to be relatively small, at 6 to 28 times more
massive than Earth. Its wide orbit means it is moving slowly around
its host star. If the suspected planet were orbiting in our solar
system, it would be roughly twice Pluto's distance from the sun.

Planets are thought to form over tens of millions of years. The
buildup is slow, but persistent as a budding planet picks up dust,
rocks, and gas from the protoplanetary disk. A planet 7.5 billion
miles from its star should 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 the deficiency of material in the disk.
Jupiter is 500 million miles from the sun and it formed in about 10
million years.

TW Hydrae is only 8 million years old, making it an unlikely star to
host a planet, according to this theory. There has not been enough
time for a planet to grow through the slow accumulation of smaller
debris. Complicating the story further is that TW Hydrae is only 55
percent as massive as our sun.

"It's so intriguing to see a system like this," said John Debes of the
Space Telescope Science Institute in Baltimore, Md. Debes leads a
research team that identified the gap. "This is the lowest-mass star
for which we've observed a gap so far out."

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 said. "That might add to planet formation theories as to how
you can actually form a planet very far out."

The TW Hydrae disk also lacks large dust grains in its outer regions.
Observations from the Atacama Large Millimeter Array in Chile show
dust grains roughly the size of a grain of sand are not present
beyond about 5.5 billion miles from the star, just short of the gap.

"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 said.

The team used Hubble's Near Infrared Camera and Multi-Object
Spectrometer (NICMOS) to observe the star in near-infrared light. The
researchers then compared the NICMOS images with archival Hubble data
and optical and spectroscopic observations from Hubble's Space
Telescope Imaging Spectrograph (STIS). Debes said researchers see the
gap at all wavelengths, which indicates it is a structural feature
and not an illusion caused by the instruments or scattered light.

The team's paper will appear online June 14 in The Astrophysical
Journal.

For images, illustrations, and more information about TW Hydrae,
visit:

http://hubblesite.org/news/2013/20

For more information about NASA's Hubble Space Telescope, visit:

http://www.nasa.gov/hubble


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