Baby 'Planemos' Can Be Born As Twins

"This is a truly remarkable pair of twins -- each weighing some hundred times less than our sun," says Ray Jayawardhana, an associate professor of astronomy and astrophysics at the University of Toronto. "Their mere existence is a surprise, and their origin and fate a bit of a mystery."

Jayawardhana and Valentin D. Ivanov of the European Southern Observatory (Chile) report the discovery in the August 3 issue of Science Express, the rapid online publication service of the journal Science.

The researchers discovered the companion candidate in an optical image taken with the European Southern Observatory's 3.5-meter New Technology Telescope on La Silla, Chile, and investigated it further with optical spectra and infrared images obtained with ESO's 8.2-meter Very Large Telescope on Paranal, Chile. These followup observations confirmed that both objects are young, at the same distance, and much too cool to be stars. By comparing the companion to widely used theoretical models, Jayawardhana and Ivanov estimate that it weighs about seven times as much as Jupiter, while the primary planemo is an estimated 14 times Jupiter's mass. The newborn pair, barely a million years old, are separated by about six times the distance between the sun and Pluto, and are located in the Ophiuchus star-forming region approximately 400 light years away.

"Roughly half of all sun-like stars, and about a sixth of brown dwarfs, come in pairs," says Jayawardhana. Brown dwarfs are 'failed stars' that weigh less than 75 Jupiter masses and are unable to sustain nuclear fusion. "Oph 162225-240515, or Oph1622 for short, is the first planemo to be resolved into a double."

The existence of this wide pair poses a challenge to a popular theory which suggests that brown dwarfs and planemos are embryos ejected from multiple proto-star systems. Since the two objects in Oph1622 are so far apart, and only weakly bound to each other by gravity, they would not have survived such a chaotic birth.

Planets are thought to form out of disks of gas and dust that surround stars, brown dwarfs and even some planemos. The researchers think that these planemo twins formed together out of a contracting gas cloud that fragmented, like a miniature stellar binary.

"We are resisting the temptation to call it a `double planet' because this pair probably didn't form the way that planets in our solar system did," says Ivanov. "Now we're curious to find out whether such pairs are common or rare. The answer could shed light on how free-floating planetary-mass objects form."