Everything we know about the formation of solar systems might be wrong, says professor Jian Ge and postdoc Bo Ma of UF Astronomy. They've discovered the first "binary–binary", or two massive companions around one star in a close binary system — one so-called giant planet (12 times the mass of Jupiter called MARVELS-7a) and one brown dwarf, or "failed star" with 57 times the mass of Jupiter, called MARVELS-7b. The findings were published October in The Astronomical Journal.
The binary system HD 87646's primary star is 12 percent more massive than our sun, yet is only 22 astronomical units away from its secondary, a star about 10 percent less massive than our sun, roughly the distance between the sun and Uranus in our solar system. An astronomical unit is the mean distance between the center of the Earth and our sun, but in cosmic terms, is a relatively short distance. Within such a short distance, two giant companions are orbiting the primary star at about 0.1 and 1.5 astronomical units away. For such large companion objects to be stable so close together defies our current popular theories on how solar systems form.
Artist's impression of the giant planet and brown dwarf system inside the HD 87646 binary system. (T. Riecken)
The planet hunting Doppler instrument, W.M. Keck Exoplanet Tracker, or KeckET, developed by a team led by Ge at the renowned Sloan Digital Sky Survey (SDSS) telescope at Apache Point Observatory in New Mexico, is unusual in that it can simultaneously observe five dozens of celestial bodies. "A typical [Doppler] project involves one object, one planet," explains Ge. He says this discovery would not have been possible without a multiple-object Doppler measurement capability such as KeckET to search for a large number of stars to discover a very rare system like this one. The survey of HD 87646 occurred in 2006 during the pilot survey of the Multi-object APO Radial Velocity Exoplanet Large-area Survey (MARVELS) of the SDSS-III program, and Ge led the MARVELS survey from 2008 to 2012. It has taken eight years of follow-up data collection through collaboration with over 30 astronomers at seven other telescopes around the world and careful data analysis, much of which was done by Bo Ma, to confirm what Ge calls a "very bizarre" finding.
Astronomers believe that planets in our solar system formed from a collapsed disk-like gaseous cloud, with our largest planet, Jupiter, buffered from smaller planets by the asteroid belt. In HD 87646, the two giant companions are close to the minimum mass for burning deuterium and hydrogen, meaning that they have accumulated far more dust and gas than what a typical collapsed disk-like gaseous cloud can provide. They were likely formed through another mechanism. The stability of the system despite such massive bodies in close proximity raises new questions about how protoplanetary disks form.
Ge hails from Beijing Astronomical Observatory, now called National Astronomical Observatory of China, but decided to relocate to the United States in 1992 to pursue the "astronomy dream" of bigger telescopes. After earning his PhD at the University of Arizona in 1998, he worked at Lawrence Livermore National Lab as a postdoc staff member in 1998-2000, at Penn State as an assistant professor in 2000-2004 before accepting a tenured professor position at UF in 2004. Bo Ma arrived in 2009 to work on the project with Ge, who emailed him to recruit him — otherwise, he would have gone elsewhere, he says. He previously studied in China, where "my brilliant [high school] teachers had Kip Thorne books on hand," which sparked his interest in how the universe came to be. After receiving his PhD last year, he stayed to analyze the data collected about HD 87646, which he calls "a good place to be." The team will continue to analyze data from the MARVELS survey. Ge and Ma will also be using UF's own 50-inch automatic telescope at Mt. Lemmon and the TOU very high-resolution optical spectrograph built by Ge's team, funded by the Dharma Endowment Foundation, to look for habitable exoplanets similar to Earth. "We are the frontier in the world," says Ge.
The two consider astronomy to be a great opportunity for public outreach to support the STEM fields, especially in light of the increasingly important quest to find an alternate home for humanity.
