Download Planet-like Companion to a Brown Dwarf

National Aeronautics and Space Administration
Planet-like Companion
to a Brown Dwarf
Taken from:
Hubble 2010: Science Year in Review
Produced by NASA Goddard Space Flight Center
and the Space Telescope Science Institute.
The full contents of this book include Hubble science articles, an overview of
the telescope, and more. The complete volume and its component sections are
available for download online at:
www.hubblesite.org/hubble_discoveries/science_year_in_review
HUBBLE 2010: SCIENCE YEAR IN REVIEW
Planet-like Companion to a Brown Dwarf
A planet-like object circling a brown dwarf has been discovered that seems to contradict the latest theories on planet
formation. Estimated to be approximately seven times the mass of Jupiter, it is the appropriate size to be described as
a large planet. But the object, known as 2M J044144 B, is believed to have formed in less than 1 million years—the
same approximate age of its brown dwarf—and much faster than the predicted time needed to build planets. Astronomers
estimated the object’s age from its temperature and brightness and by knowing that it is the same age or younger than the
brown dwarf, known as 2M J044144 A. They determined the brown dwarf’s age by applying models of how brown dwarfs
cool. They also know that it resides in a star-forming region where the stars are an average of one million years old. The
mysterious object orbits the nearby brown dwarf at a separation of approximately 2.25 billion miles, which is between the
distances of Saturn and Uranus from the Sun.
Kevin Luhman of Pennsylvania State University, his graduate student Kamen Todorov, and Kim McLeod of Wellesley College
used Hubble and the Gemini Observatory in Hawaii to image the brown dwarf’s companion directly. They uncovered the
companion in a survey of 32 young brown dwarfs in the Taurus star-forming region. Brown dwarfs are objects that typically
are tens of times the mass of Jupiter and are too small to sustain nuclear fusion, inhibiting them from becoming stars.
Much discussion has recently occurred in the context of the Pluto debate over how small an object can be and still be called
a planet. The discovery of 2M J044144 B raises questions at the opposite end of the size spectrum: How large can an object
be and still be called a planet rather than a brown dwarf? The mass of 2M J044144 B is within the range of masses found
for the orbiting bodies in many known extrasolar planetary systems—less than 15 Jupiter masses. But should it be called a
planet if it didn’t form by the agglomeration of material in a debris disk around a star? This is, after all, the currently accepted
understanding of how planets form.
This artist’s concept of the binary system 2M J044144 shows the primary brown dwarf, 20 times the mass of Jupiter (at left), and its
companion, which is estimated to be about seven times the mass of Jupiter (at right).
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On the left, the young brown dwarf 2M J044144 A has a companion object, 2M J044144 B, at the 8 o’clock position, which is estimated to be five to
ten times the mass of Jupiter. In the picture on the right, the light from the brown dwarf has been subtracted to provide a clearer view of the companion
object. The companion may be a very small brown dwarf or a large planet, depending on how it formed. Images were taken with Hubble’s Wide Field
Planetary Camera 2 to track the motion of the two objects to determine that they actually do travel across space together. Additional observations were
done with the Gemini North telescope on Mauna Kea, Hawaii.
There are presently three identified formation scenarios for such an object. In the first—the core accretion model—dust
orbiting the star slowly clumps to form a rocky planet ten times larger than Earth, which then collects a large gaseous
envelope. In the second—the disk instability model—a lump of gas in the disk quickly collapses to form an object the size
of a giant gas planet. In the third and distinctly different one—the cloud fragmentation model—a companion forms directly
from the collapse of a vast cloud of gas and dust in the same manner as its star (or brown dwarf) rather than forming in a
disk. If this is what actually took place, then the discovery of 2M J044144 B demonstrates that planetary-mass bodies can
be made through the same mechanism that forms stars.
In this case, the cloud fragmentation model is the likely scenario for three reasons. First, 2M J044144 B is too young to
have formed by core accretion, which is a very slow process. Second, calculations indicate that the central brown dwarf in
this system probably did not contain enough material to make an object with a mass of five to ten Jupiter masses via disk
instability. Third, another nearby star contains a small red star, 2M J044145 A, and a brown dwarf, 2M J044145 B.
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Core Accretion Model
Planet
agglomerates
from dust
Disk Instability Model
Central
star
Dust disk
Clump of gas
condenses in
circumstellar
disk
Cloud Fragmentation Model
Clouds
condense to
form planets
This graphic shows the three possible formation scenarios for the planet-like companion. If the last scenario is correct, then this discovery demonstrates
that planetary-mass bodies can be made through the same mechanism that builds stars.
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Taken together, these four bodies closely resemble a quadruple star system, suggesting that all of the components formed
through cloud fragmentation and collapse.
Clearly, the 2M J044144 system provides astronomers several good reasons to believe that planetary-mass companions
can form through cloud collapse and fragmentation in addition to the more conventional but much slower disk-accretion
processes. Further Hubble and ground-based observations of such bodies should help theorists refine their models of
planetary formation and thereby contribute important information to distinguish over time the boundary between planets
and brown dwarfs.
Further Reading
“Astrophysics: The Odd Couple.” Nature 464, no. 7291 (April 15, 2010): 961.
Basri, G. and M. Brown. “Planetesimals to Brown Dwarfs: What is a Planet?” Annual Review of Earth and Planetary Sciences 34
(2006): p.193–216.
Berardelli, P. “Scienceshot: A Brown Dwarf’s Mysterious Companion.” ScienceNOW, April 6, 2010.
http://news.sciencemag.org/sciencenow/2010/04/scienceshot-a-brown-dwarfs-myste.html (accessed January 5, 2011).
Chabrier, G., et al. “Gaseous Planets, Protostars, and Young Brown Dwarfs: Birth and Fate.” In Protostars and Planets V.
Edited by B. Reipurth, D. Jewitt, and K. Keil, 623–638. Tucson, AZ: University of Arizona Press, 2007.
Luhman, K. L. , et al. “The Formation of Brown Dwarfs: Observations.” In Protostars and Planets V.
Edited by B. Reipurth, D. Jewitt, and K. Keil, 443–457. Tucson, AZ: University of Arizona Press, 2007.
Mohanty, S. “The Mystery of Brown Dwarf Origins.” Scientific American 294, no. 1 (January 2006): 38–45.
Todorov, K., et al. “Discovery of a planetary-mass companion to a brown dwarf in Taurus.” Astrophysical Journal Letters 714, no. 1
(May 1, 2010): L84–L88.
Werner, M. W. “Improbable planets.” Scientific American 300, no. 6 (June 2009): 38–44.
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Dr. Kevin Luhman has used a variety of optical and infrared telescopes to study brown dwarfs and circumstellar
disks. Born in Kansas, he earned both his bachelor of arts in astronomy and his bachelor of science in physics
from the University of Texas in 1993 and a doctorate in astronomy from the University of Arizona in 1998. Dr.
Luhman was a postdoctoral fellow at the Harvard-Smithsonian Center for Astrophysics and is now a professor
of astronomy and astrophysics at Pennsylvania State University. Using Hubble and other facilities such as the
Spitzer Space Telescope, he continues to search for the smallest bodies that are able to form in isolation and
as widely separated companions.
Dr. Kim Katris McLeod is a professor of astronomy at Wellesley College in Massachusetts. Her interests
include imaging very distant quasars to see how their host galaxies grow through cosmic time, and searching
for disks and giant planets around young stars and brown dwarfs. Her astronomical journey started in her
home state of Delaware, where she grew up delighting in constellations and calculus. She earned her bachelor
of arts degree in physics from Cornell University in 1988 and her doctorate in astronomy from the University
of Arizona in 1994. Before joining the Wellesley faculty, she worked as a post-doc at the Harvard-Smithsonian
Center for Astrophysics. She has also been a Radcliffe Institute Fellow.
As an undergraduate student, Kamen Todorov studied a transiting extra solar planet with data from the Spitzer
Space Telescope under the supervision of Dr. Drake Deming. He was born in Bulgaria and earned a bachelor
of arts degree in astrophysics at Connecticut College in 2008. Mr. Todorov is now a graduate student in
astronomy and astrophysics at Pennsylvania State University.
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