Planet-like Companion to a Brown Dwarf

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1 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:

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3 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 J 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 J 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 J 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 J 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 J 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). 99

4 On the left, the young brown dwarf 2M J A has a companion object, 2M J 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 J 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 J 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 J A, and a brown dwarf, 2M J B. 100

5 Planet agglomerates from dust Core Accretion Model Central star Disk Instability Model 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. 101

6 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 J 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 (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 Berardelli, P. Scienceshot: A Brown Dwarf s Mysterious Companion. ScienceNOW, April 6, (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, Tucson, AZ: University of Arizona Press, 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, Tucson, AZ: University of Arizona Press, Mohanty, S. The Mystery of Brown Dwarf Origins. Scientific American 294, no. 1 (January 2006): 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):

7 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 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 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 Mr. Todorov is now a graduate student in astronomy and astrophysics at Pennsylvania State University. 103