"Long before it's in the papers"
January 27, 2015

RETURN TO THE WORLD SCIENCE HOME PAGE


“Free-floating planet” photographed for first time

Oct. 9, 2013
Courtesy of the Max Planck Institute for Astronomy
and World Science staff

As­tro­no­mers say they’ve pho­to­graphed a free-float­ing plan­et, a type of body the­o­rized for years to ex­ist but nev­er im­aged di­rect­ly. 

But oth­er new find­ings are al­so blur­ring the bound­aries be­tween such plan­ets, and stars. 

Sci­en­tists are in­creas­ingly say­ing there may be no fun­da­men­tal dif­fer­ence be­tween a large free-float­ing plan­et and a small, “failed” star. A study be­ing re­leased at the same time as the new plan­et pho­to con­cludes that at least some free-float­ing plan­ets form much more like stars do than like plan­ets, as tra­di­tion­ally con­sid­ered, do.

Credit: PS1 Science Collaboration


It used to be sim­ple to de­scribe the dif­fer­ence be­tween a plan­et and a star. On the one hand, there were stars: gi­gantic nu­clear fur­naces, emit­ting lots of light. On the oth­er, there were plan­ets, much smaller, cir­cling a host star and re­flect­ing its light. The forma­t­ion pro­cesses were con­sid­ered dif­fer­ent, too. Stars formed from the col­lapse of gi­gantic clouds of gas; plan­ets formed in disks of gas and dust around their fledg­ling host stars.

As­tro­no­mers have long rec­og­nized one slight am­bigu­ity. There is an in­ter­me­diate ob­ject called a brown dwarf, heav­i­er than a plan­et, but not mas­sive enough to be­come a star. That would re­quire an energy-gen­er­a­t­ing pro­cess, nu­clear fu­sion, to ig­nite at the ob­ject’s co­re.

Some of the new find­ings blur the plan­et-star bound­aries much fur­ther.

In one stu­dy, as­tro­no­mers led by Mi­chael Liu of the Uni­vers­ity of Ha­waii pho­tographed what they called “an ex­ot­ic young ob­ject” weigh­ing the equiv­a­lent of six Ju­pi­ters, and float­ing in space alone—no host star. The ob­ject, dubbed PSO J318.5-22, lies in the di­rec­tion of the con­stella­t­ion Cap­ri­cor­nus, and an es­ti­mat­ed 80 light-years away from Earth. A light-year is the dis­tance light trav­els in a year.

Its prop­er­ties are si­m­i­lar to those of gi­ant gas plan­ets found or­bit­ing around young stars, in­clud­ing those few that have been pho­tographed, said Liu and col­leagues, who us­ing the Pan-STARRS 1 tel­e­scope on Ha­waii. The fact that the ob­ject has no host star makes it eas­i­er to see, they add, as the gently glow­ing plan­et is more clearly vis­i­ble against a dark back­ground than against star­light.

This will make it “much eas­i­er for us to stu­dy. It is go­ing to pro­vide a won­der­ful view in­to the in­ner work­ings of gas-gi­ant plan­ets like Ju­pi­ter in an early phase of their evo­lu­tion,” said Niall Dea­con of the Max Planck In­sti­tute for As­tron­o­my in Hei­del­berg, Ger­ma­ny, a co-author of the re­port on the stu­dy. 

An artist's impression of the "free-floating plan­et" OTS44 as it forms by gath­e­ring mat­erial from the sur­round­ing cloud.
(Cre­dit: A. M. Quetz)


The ob­ject is al­so one of the light­est free-float­ing worlds known, per­haps the light­est, the as­tro­no­mers said. But how does such a thing come in­to ex­istence? Can free-float­ing plan­ets and brown dwarfs form like or­di­nary stars? A study of a dif­fer­ent ob­ject, pub­lished at the same time by anoth­er group of as­tro­no­mers led by the In­sti­tute’s Viki Jo­er­gens, sug­gests they do.

They stud­ied an ob­ject called OT­S44, which an es­ti­mat­ed two mil­lion years old—a ba­by in terms of star or plan­et ages—and around twice the weight of the pho­tographed plan­et. It’s al­so float­ing through space with­out a close com­pan­ion, but in a par­tic­u­larly in­ter­est­ing part of space: the Cha­me­le­on star-forming re­gion in the South­ern con­stella­t­ion Cha­me­le­on. That’s a lit­tle over 500 light-years from Earth, where many new stars are born from the col­lapse of thick gas-and-dust clouds.

OT­S44 sits in the mid­dle of a disk of gas and dust, which seems to be feed­ing it new ma­te­ri­al, Jo­er­gens and col­leagues found. That’s pret­ty much how stars form. “Our ob­serva­t­ions show that, even now, there is still gas fall­ing on­to OT­S44,” she said. Oth­er da­ta in­di­cates the feed­er disk is sub­stantial, she added, a fur­ther sign of si­m­i­lar­ity in the forma­t­ion pro­cess.

Such ob­jects fall in­to no ex­isting cat­e­gories clear­ly. Sol­i­tary plan­ets or ex­tremely low-mass brown dwarf­s—if you want to play it safe, you can talk, more gen­er­al­ly, about free-float­ing plan­etary-mass ob­jects, some as­tro­no­mers sug­gested. “This is anoth­er in­dica­t­ion that our tra­di­tion­al cat­e­gories of plan­ets and stars, which are based on mass [or “weight”] val­ues, tell us noth­ing about the in­ner struc­ture or the forma­t­ion his­to­ry of these ob­jects,” said Hu­bert Klahr of the In­sti­tute, an ex­pert in sim­ula­t­ions of star and plan­et forma­t­ion, who was­n’t in­volved in the re­search.

The findings on PSO J318-22 are being pub­lished by the jour­nal Astro­physical Jour­nal Let­ters; those re­gard­ing OTS44, by Astro­nomy & Astro­physics Let­ters.


* * *

Send us a comment on this story, or send it to a friend

[../index-contents.html]

Astronomers say they’ve for the first time photographed a free-floating planet, a type of body long theorized to exist but never imaged directly. But other new findings are also blurring the boundaries between such planets, and stars. Scientists are increasingly claiming there may be no fundamental difference between a large free-floating planet and a small, “failed” star. A study being released at the same time as the new planet photo concludes that at least some free-floating planets form much more like stars do than like planets, as traditionally considered, do. It used to be simple to describe the difference between a planet and a star. On the one hand, there were stars: gigantic nuclear furnaces, emitting lots of light. On the other, there were planets, much smaller, circling a host star and reflecting its light. The formation of the two types of objects was considered different, too. Stars formed from the collapse of gigantic clouds of gas; planets formed in disks of gas and dust around their fledgling host stars. Astronomers have recognized one slight ambiguity for years. There is an intermediate object called a brown dwarf. It’s heavier than a planet—but not massive enough to become a star, which would require an energy-generating process, nuclear fusion, to ignite at the object’s core. Some of the new findings blur the planet-star boundaries much further. In one study, astronomers led by Michael Liu of the University of Hawaii photographed what they called “an exotic young object” weighing the equivalent of six Jupiters, and floating in space alone—no host star. The object, dubbed PSO J318.5-22, lies in the direction of the constellation Capricornus, and an estimated 80 light-years away from Earth. A light-year is the distance light travels in a year. Its properties are similar to those of giant gas planets found orbiting around young stars, including those few that have been photographed, said Liu and colleagues, who using the Pan-STARRS 1 telescope on Hawaii. The fact that the object has no host star makes it easier to see, they add, as the gently glowing planet is more clearly visible against a dark background than against starlight. It’s “not orbiting a star so it will be much easier for us to study. It is going to provide a wonderful view into the inner workings of gas-giant planets like Jupiter in an early phase of their evolution,” said Niall Deacon of the Max Planck Institute for Astronomy in Heidelberg, Germany, a co-author of the study. The object is also one of the lightest free-floating worlds known, perhaps the lightest, the astronomers said. But how does such a thing come into existence? Can free-floating planets and brown dwarfs form like ordinary stars? A study of a different object, published at the same time by another group of astronomers led by the Institute’s Viki Joergens, suggests they do. They studied an object called OTS44, which an estimated two million years old—a baby in terms of star or planet ages—and around twice the weight of the photographed planet. It’s also floating through space without a close companion, but in a particularly interesting part of space: the Chameleon star-forming region in the Southern constellation Chameleon. That’s a little over 500 light-years from Earth, where many new stars are born from the collapse of thick gas-and-dust clouds. OTS44 sits in the middle of a disk of gas and dust, which seems to be feeding it new material, Joergens and colleagues found. That’s pretty much how stars form. “Our observations show that, even now, there is still gas falling onto OTS44,” she said. Other data indicates the feeder disk is substantial, she added, a further sign of similarity in the formation process. Such objects fall into no existing categories clearly. Solitary planets or extremely low-mass brown dwarfs—if you want to play it safe, you can talk, more generally, about free-floating planetary-mass objects, some astronomers suggested. “This is another indication that our traditional categories of planets and stars, which are based on mass [or “weight”] values, tell us nothing about the inner structure or the formation history of these objects,” said Hubert Klahr of the Institute, an expert in simulations of star and planet formation, who wasn’t involved in the research.