When I was young, I was told that we would never be able to image a planet that is orbiting another star. Yet in November 2008, Hubble took the first visible light image of a planet orbiting around the star Fomalhaut, which is approximately 25 light years from Earth. At the same time, astronomers using the Keck and Gemini telescopes imaged three orbiting companions to star HR8799, which is approximately 140 light years from Earth. Now, Hubble has done it again, and has either imaged a large gas giant orbiting a brown dwarf, or has seen a binary brown dwarf system.
A brown dwarf is an astronomical object that has too little mass to sustain hydrogen fusion. This places an upper limit on its size about about 75 times the mass of Jupiter. The lower limit of a brown dwarf is about 13 times the mass of Jupiter, which is the lower limit for fusing deuterium. With that definition out of the way, the new object is likely a large gas planet since it is calculated to be the size of 5 to 10 times the mass of Jupiter.
The problem is that the brown dwarf appears to be less than 1 million years old. If that is true, then how did the planet form? Scientists have three theories for planet formation. The first is that the dust in a disk orbiting a start slowly accumulates to for a rocky planet, which then accumulates a large amount of gas. The problem is that this route to large planet formation takes a long time. The second theory is that circumstellar disk is unstable, and a clump of gas collapses quickly to form the planet. The third theory is that the cloud of gas that collapses to form the brown dwarf has fragmented to form its binary companion. These three theories are shown in the second graphic to the right.
This is a Hubble Space Telescope image of young brown dwarf 2M J044144. It has a companion object at the 8 o’clock position that is estimated to be 5-10 times the mass of Jupiter. In the right panel, the light from the brown dwarf has been subtracted to provide a clearer view of the companion object. The separation of the companion corresponds to 1.4 billion miles at the distance of the Taurus star-forming region, which is only about 1 million years old. 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 see if they actually do travel across space together. Additional observations were done with the Gemini North telescope on Mauna Kea, Hawaii.
