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Free-floating planets can be born free, study says

Aug. 22, 2013
World Science staff

Lit­tle round, cold clouds in space have right char­ac­ter­is­tics to form plan­ets with no par­ent star, as­tro­no­mers are re­port­ing.

Ob­serva­t­ions made with tele­scopes at the Chal­mers Uni­vers­ity of Tech­nol­o­gy in Swe­den sug­gest that not all free-float­ing plan­ets were thrown out of ex­ist­ing plan­e­tary sys­tems: they can be born free.

Globulettes in the Rosette nebula (circled), accom­pan­ied by a graph de­scrib­ing the light spec­trum of one of them. (Cred­it: ESO/M. Mä­ke­lä)


Pre­vi­ous re­search in­di­cates there may be as many as 200 bil­lion free-float­ing plan­ets in our gal­axy. Sci­en­tists have be­lieved such “rogue plan­ets,” which don’t or­bit a star, must have been ejected from ex­ist­ing plan­e­tary sys­tems. The new find­ings sug­gest oth­er­wise.

As­tro­no­mers from Swe­den and Fin­land used sev­er­al tele­scopes to ob­serve the Ro­sette Neb­u­la, a huge cloud of gas and dust 4600 light-years from Earth in the con­stella­t­ion Mo­noc­er­os, the Un­icorn (a light-year is the dis­tance light trav­els in a year). They col­lect­ed ob­serva­t­ions in var­i­ous wave­lengths, or “col­ors,” of light, wave­lengths not vis­i­ble to the eye. 

“The Ro­sette Neb­u­la is home to more than a hun­dred of these ti­ny clouds – we call them glob­ulettes,” said Gösta Gahm, as­tron­o­mer at Stock­holm Uni­vers­ity, who led the proj­ect. “They are very small, each with di­am­e­ter less than 50 times the dis­tance be­tween the Sun and Nep­tune.”

“We found that the glob­ulettes are very dense and com­pact, and many of them have very dense cores,” added team mem­ber Ca­ri­na Pers­son, as­tron­o­mer at Chal­mers. “That tells us that many of them will col­lapse un­der their own weight and form free-float­ing plan­ets. The most mas­sive of them can form so-called brown dwarfs,” a type of “failed” star some­where be­tween plan­ets and normal stars in size.

The study in­di­cates the ti­ny clouds are mov­ing out­wards through the Ro­sette Neb­u­la at about 80,000 km (50,000 miles) per hour. “We think that these small, round clouds have bro­ken off from tall, dusty pil­lars of gas which were sculpted by the in­tense radia­t­ion from young stars. They have been ac­cel­er­ated out from the cen­ter of the neb­u­la thanks to pres­sure from radia­t­ion from the hot stars in its cen­ter,” said Uni­vers­ity of Hel­sin­ki as­tron­o­mer Minja Mäkelä.

Dur­ing the his­to­ry of the Milky Way, count­less mil­lions of neb­u­lae like the Ro­sette have bloomed and fad­ed away, the sci­en­tists said, and in all of these, many glob­ulettes would have formed.

“If these ti­ny, round clouds form plan­ets and brown dwarfs, they must be shot out like bul­lets in­to the depths of the Milky Way,” said Gahm. “There are so many of them that they could be a sig­nif­i­cant source of the free-float­ing plan­ets that have been dis­cov­ered in re­cent years.”

The research appears in the July issue of the jour­nal Astron­omy & Astro­physics.


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Homepage image: the Onsala Space Observatory at Chalmers University of Technology, Sweden

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Little round, cold clouds in space have right characteristics to form planets with no parent star, astronomers are reporting. Observations made with telescopes at the Chalmers University of Technology in Sweden suggest that not all free-floating planets were thrown out of existing planetary systems: they can be born free. Previous research indicates there may be as many as 200 billion free-floating planets in our galaxy. Scientists have believed such “rogue planets,” which don’t orbit a star, must have been ejected from existing planetary systems. The new findings suggest otherwise. Astronomers from Sweden and Finland used several telescopes to observe the Rosette Nebula, a huge cloud of gas and dust 4600 light years from Earth in the constellation Monoceros, the Unicorn. They collected observations in various wavelengths, or “colors,” of light, wavelengths not visible to the eye. “The Rosette Nebula is home to more than a hundred of these tiny clouds – we call them globulettes,” said Gösta Gahm, astronomer at Stockholm University, who led the project. “They are very small, each with diameter less than 50 times the distance between the Sun and Neptune. Previously we were able to estimate that most of them are of planetary mass, less than 13 times Jupiter’s mass. Now we have much more reliable measures of mass and density for a large number of these objects, and we have also precisely measured how fast they are moving relative to their environment. “We found that the globulettes are very dense and compact, and many of them have very dense cores. That tells us that many of them will collapse under their own weight and form free-floating planets. The most massive of them can form so-called brown dwarfs,” a type of “failed” star, said team member Carina Persson, astronomer at Chalmers University of Technology. Brown dwarfs are bodies somewhere between planets and stars in size. The study indicates the tiny clouds are moving outwards through the Rosette Nebula at about 80,000 km (50,000 miles) per hour. “We think that these small, round clouds have broken off from tall, dusty pillars of gas which were sculpted by the intense radiation from young stars. They have been accelerated out from the center of the nebula thanks to pressure from radiation from the hot stars in its center,” said University of Helsinki astronomer Minja Mäkelä. During the history of the Milky Way, countless millions of nebulae like the Rosette have bloomed and faded away, the scientists said, and in all of these, many globulettes would have formed. “If these tiny, round clouds form planets and brown dwarfs, they must be shot out like bullets into the depths of the Milky Way,” said Gahm. “There are so many of them that they could be a significant source of the free-floating planets that have been discovered in recent years.”