Credit: University of Florida

Discovery of First Binary-Binary Calls Theory on Formation of Solar System into Question

Astronomy professor at University of Florida Jian Ge and his postdoctoral student Bo Ma, believe that everything we know about the formation of solar systems might be wrong. They have discovered two massive companions around one star in a close binary system, one a so-called giant planet and the other a brown dwarf. This configuration is known as a “binary–binary” and is the first of its kind discovered. The giant planet is called MARVELS-7a and it is 12 times the mass of Jupiter, while the brown dwarf, called MARVELS-7b, has 57 times the mass of Jupiter.

Astronomers think that the planets in our solar system were formed from a collapsed disk-like gaseous cloud.  Jupiter, our largest planet is believed to have been buffered from smaller planets by the asteroid belt. In HD 87646, the newly discovered binary system, the two massive companions are close to the minimum mass for burning hydrogen and deuterium. This means that they have gathered far more gas and dust than what a typical collapsed disk-like gaseous cloud can provide. This leads to the theory that they were likely formed through a different mechanism. The stability of the system despite such enormous bodies close to each other also raises new questions about how protoplanetary disks are formed.

An astronomical unit (AU) is a relatively short distance in cosmic terms. It is defined as the mean distance between our Sun and the center of the Earth. Although HD 87646’s primary star is 12 percent more massive than our sun, it is only 22 astronomical units away from its secondary. This is approximately the distance between our Sun and Uranus. The secondary star is about 10 percent less massive than our Sun. The two huge companions are orbiting the primary star at about 0.1 and 1.5 astronomical units away within this short distance. Our current popular theories on how solar systems come into existence are defied by such large companion objects being stable while so close together.

The W.M. Keck Exoplanet Tracker, or KeckET, is a planet hunting Doppler instrument. It is unusual in that it can observe dozens of celestial bodies at the same time. A team led by Ge at the Sloan Digital Sky Survey telescope at Apache Point Observatory in New Mexico, originally developed KeckET. Ge notes that without a multiple object Doppler measurement capability such as KeckET to search for a large number of stars, the discovery of a rare system like this one would not have been possible.

In 2006, a survey of HD 87646 was done as part of the pilot survey of the Multi object APO Radial Velocity Exoplanet Large area Survey (MARVELS) of the SDSS-III program. Ge led the MARVELS survey lasting from 2008 to 2012. Ge’s unusual finding was only verified eight years later through follow up data collection. The data collection and analysis was done by more than 30 astronomers collaborating at seven other telescopes around the world. Much of this work was done by Bo Ma.

The full study was published in the Astronomical Journal.