Oddball Objects: Neither Stars or Planets
An artists illustration of the newly discovered double planemo system.
Credit: ESO

A newly spotted pair of planet-sized objects floating far away from any star has astronomers puzzling over how such a bizarre system could have formed.

Using the European Southern Observatory's telescopes in Chile, astronomers have spotted a seven-Jupiter mass object paired to another 14-Jupiter-mass companion. Instead of orbiting around a star, however, the two planetary mass objects, or "planemos," are circling each other.

Planemos are objects similar to brown dwarfs, failed stars too small to sustain the nuclear reactions required for stellar ignition. But at only a few times more massive than Jupiter, they resemble planets more than stars.

The finding, detailed in the August 4 issue of the journal Science, places strong constraints on planemo formation theories and could shed light on how the strange objects form.

A first

About half of all Sun-like stars have stellar companions. About a sixth of all brown dwarfs are also paired. But the new system, called Oph 162225-240515, or Oph1622 for short, is the only known planemo binary.

"Recent discoveries have revealed an amazing diversity of worlds out there. Still, the Oph1622 pair stands out as one of the most intriguing, if not peculiar," said study team member Ray Jayawardhana of the University of Toronto in Canada.

The pair is barely a million years old and located in the Ophiuchus star-forming region, about 400 light years away. The objects are separated by about six times the distance between the Sun and Pluto. Infrared observations by NASA's Spitzer Space Telescope suggest that at least one, and possibly both, of the planemos are ringed by a disk of gas and dust.

Light and color analysis of the planemos strongly suggests they were born at the same time but scientists aren't sure how they formed. The answer to this latter question could determine whether or not the objects are classified as planets.


Are they planets?

If size were the only criteria for being called a planet, then most of the few dozen planemos discovered thus far would certainly qualify. Astronomers sometimes draw the cut off line for whether or not an object is a planet at 13-Jupiter-masses. Objects more massive than this can burn a form of hydrogen called deuterium for a short time early in their lives while those below this limit cannot.

But some scientists argue that planethood shouldn't be bestowed based solely on mass, and that how an object was created should also be considered. Most known planets, including those in our solar system, form out of discs of gas and dust surrounding stars or brown dwarfs. As far as scientists can tell, the new planemo pair did not form this way.

Instead, scientists think the twins formed the same way that many binary star systems do, when a contracting gas cloud splits in two before condensing into a stellar core.

"We are resisting the temptation to call it a 'double planet' because this pair probably didn't form in the way that planets in our solar system did," said ESO astronomer and study team member Valentin Ivanov.

A weak link

According to another theory, planemos are embryos ejected from nascent stellar nurseries. However, the two objects in Oph1622 are so far apart and so weakly bound to each other by gravity that they would not have survived such an ejection, the researchers say. Their connection is so tenuous, in fact, that a passing star or brown dwarf could permanently separate the two objects.

The researchers say the next step is to determine if binary planemos are the rule or whether the new discovery is a rare exception.

"The answer could shed light on how free-floating planetary mass objects form," Ivanov said.