'Super-Jupiter' discovered around massive star
Using infrared data from the Subaru Telescope in Hawaii, astronomers have discovered a "super-Jupiter" around the bright star Kappa Andromedae, which now holds the record for the most massive star known to host a directly imaged planet or lightweight brown dwarf companion.
Designated Kappa Andromedae b (Kappa And b, for short), the new object has a mass about 12.8 times greater than Jupiter's. This places it teetering on the dividing line that separates the most massive planets from the lowest-mass brown dwarfs. That ambiguity is one of the object's charms, said researchers, who call it a super-Jupiter to embrace both possibilities.
The "super-Jupiter" circles its star at nearly twice the distance that Neptune orbits the sun and glows with a reddish color.
"According to conventional models of planetary formation, Kappa And b falls just shy of being able to generate energy by fusion, at which point it would be considered a brown dwarf rather than a planet," said Michael McElwain, a member of the discovery team at NASA's Goddard Space Flight Center in Greenbelt, Md.
"But this isn't definitive, and other considerations could nudge the object across the line into brown dwarf territory," he stated.
Massive planets slowly radiate the heat leftover from their own formation. For example, the planet Jupiter emits about twice the energy it receives from the sun. But if the object is massive enough, it's able to produce energy internally by fusing a heavy form of hydrogen called deuterium. (Stars like the sun, on the other hand, produce energy through a similar process that fuses the lighter and much more common form of hydrogen.) The theoretical mass where deuterium fusion can occur -- about 13 Jupiters -- marks the lowest possible mass for a brown dwarf.
"Kappa And b, the previously imaged planets around HR 8799 and Beta Pictoris, and the most massive planets discovered by non-imaging techniques likely all represent a class of object that formed in much the same way as lower-mass exoplanets," said lead researcher Joseph Carson, an astronomer at the College of Charleston, S.C., and the Max Planck Institute for Astronomy in Heidelberg, Germany.
"This object demonstrates that stars as large as Kappa And, with 2.5 times the sun's mass, remain fully capable of producing planets," Carson added.
Kappa And b orbits its star at a projected distance of 55 times Earth's average distance from the sun and about 1.8 times as far as Neptune; the actual distance depends on how the system is oriented to our line of sight, which is not precisely known. The object has a temperature of about 2,600 degrees Fahrenheit (1,400 Celsius) and would appear bright red if seen up close by the human eye.
Carson's team detected the object in independent observations at four different infrared wavelengths in January and July of this year.
Comparing the two images taken half a year apart showed that Kappa And b exhibits the same motion across the sky as its host star, which proves that the two objects are gravitationally bound and traveling together through space. Comparing the brightness of the super-Jupiter between different wavelengths revealed infrared colors similar to those observed in the handful of other gas giant planets successfully imaged around stars.
A paper describing the results has been accepted for publication in The Astrophysical Journal Letters and will appear in a future issue.