Tuesday, July 30, 2013

First Mini Me Solar System Found, Huge Planet Orbits Tiny Brown Dwarf

 Astronomers have found the first planet orbiting a brown dwarf star.  The system a miniature version of our own solar system,  a kind of Mini Me Solar System. 

Observations of accretion disks around young brown dwarfs have led to the speculation that they may form planetary systems similar to normal stars. While there have been several detections of planetary-mass objects around brown dwarfs (2MASS 1207-3932 and 2MASS 0441-2301), these companions have relatively large mass ratios and projected separations, suggesting that they formed in a manner analogous to stellar binaries

This artist's conception shows the relative size of a hypothetical brown dwarf-planetary system compared to our own solar system. A brown dwarf is a cool or "failed" star, which lacks the mass to ignite and shine like our Sun. NASA's Spitzer Space Telescope set its infrared eyes on an extraordinarily low-mass brown dwarf called OTS 44 and found a swirling disk of planet-building dust. At only 15 times the mass of Jupiter, OTS 44 is the smallest known brown dwarf to host a planet-forming, or protoplanetary, disk.

Image Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Astrophysicists have presented the discovery of a planetary-mass object orbiting a field brown dwarf via gravitational microlensing, OGLE-2012-BLG-0358Lb. The system is a low secondary/primary mass ratio (0.080 +- 0.001), relatively tightly-separated (~0.87 AU) binary composed of a planetary-mass object with 1.9 +- 0.2 Jupiter masses orbiting a brown dwarf with a mass 0.022 M_Sun. The relatively small mass ratio and separation suggest that the companion may have formed in a protoplanetary disk around the brown dwarf host, in a manner analogous to planets.

Brown dwarfs are dwarf stars that are less than one-tenth the mass of our sun.Astrophysical calculations show that such a small star is too small for the "sun ignition" necessary to sustain hydrogen fusion in its core. Such failed stars therefore never light up and therefore constitute a category of warm celestial bodies between stars and large gas planets.

Previously there was no evidence that the formation of brown dwarfs  also had orbiting  planets. However, this deficiency was mainly based on the fact that brown dwarfs are difficult to observe because they are extremely faint and so also any existing planets making identification with the classical methods of planet detection  even more difficult.

For this reason, astronomers used an entirely different search method, called gravitational lensing  to search for planets around a brown dwarf.  Gravitational lensing occurs when a celestial body passes in front of another and its gravity focuses the light of the background object toward Earth. This method has the advantage that it works regardless of the brightness of the body. The method was used by C. Han of the Korean Chungbuk National University and astronomers from the Max Planck Institute for Solar System Research. Their results are current published in "Microlensing Discovery of a Tight, Low Mass-ratio Planetary-mass Object around an Old, Field Brown Dwarf."

The brown dwarf that the researchers currently have examined is about 6,000 light-years away from Earth and resides in the constellation Scorpio.  The astronomers became aware of variations in brightness of the dwarf star for the first time in April 2012 and were able to ensure by further observations, it  was indeed a gravitational lens effect. 

Based on the data the scientists conclude that the brown dwarf is orbited by a planet about has twice the mass of our Jupiter and is less than one astronomical unit (AU = distance Sun-Earth) from the star it orbits. The brown dwarf itself is about ten times larger than its planetary companion. 

 Brown dwarfs (BDs) are sub-stellar objects that are too low in mass to sustain hydrogen fusion reactions in their cores. Although still a matter of debate, the most popular theory about the origin of BDs is that they form via direct collapse similar to stars, perhaps aided by turbulent fragmentation. This theory is supported by observational evidence showing that several medium-sized BDs are girdled by disks of material.  The existence of accretion disks around these failed stars naturally leads to the speculation that BDs may also harbor planetary systems analogous to
those found in abundance around stars. 

There is still debate as to whether the brown dwarf captured it planets or if they formed in a planetary disk around the star.

Dr Evil and Mini Me from the Austin Powers film from which the new solar system derives its comparison.
Credit   Austin Powers in Goldmember


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