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Lost planet, or part of vastest system?

Sept. 5, 2006
Special to World Science  

Strange new worlds that re­de­fine the word “plan­et” are con­stant­ly com­ing to light. But even as­tronomers, who are used to this, got a jolt six years ago when ap­par­ent or­phan plan­ets were an­nounced. These seem to roam the cos­mos with no star to pin them in­to an or­bit.

The Sig­ma Or­i­o­nis star clus­ter ap­pears as the bright star-like ob­ject near the bot­tom of this im­age. The clus­ter is a fa­vor­ite hunt­ing grounds of planet-searchers. The image al­so shows the fa­mous Horse­head Neb­u­la and O­ri­on's belt, part of the O­ri­on con­stel­la­tion. The belt's three stars are the bright­est in the im­age, running from the up­per right cor­ner down­ward and left­ward. Be­low the left­most of these three is the Horse­head Neb­u­la. Fol­low the di­rec­tion of the "horse's" neck and you reach Sig­ma Or­i­o­nis, to the right and slight­ly down­ward. (Cour­te­sy Dig­i­tized Sky Sur­vey, ESA/E­SO/­NASA FITS Lib­er­a­tor; Col­or Com­pos­ite: Da­vide De Mar­tin (Sky­fac­to­ry)) 


Now, four re­search­ers claim that at least one of these may not be a free-floater af­ter all. It might in­stead just be on a gra­v­i­ta­tion­al leash so long, it’s part of by far the vast­est known pla­n­e­tar­y sys­tem, do­zens of times wid­er than ours.

Not eve­ry­one is con­vinced. 

None­the­less, at a time when some sci­en­tists have be­gun ex­u­ber­ant­ly sug­gest­ing that the cos­mos is buz­z­ing with roam­ing pla­nets, the four say it may be time to step back and re­con­si­d­er what a pla­ne­tary sys­tem is.

Perhaps “not all ‘iso­la­t­ed’ pla­n­e­tar­y-mass ob­jects are real­ly iso­lat­ed,” they wrote in a pa­per to ap­pear in the re­search jour­nal As­tron­o­my & As­tro­phys­ics.

But they ad­mit they’ve shown on­ly that a can­di­date plan­et and its star are close enough to or­bit each oth­er, not that they do. That’s “pretty spec­u­la­tive,” wrote Phil Lu­cas of the Uni­ver­si­ty of Hert­ford­shire in Hat­field, U.K., co-dis­co­verer of the first re­por­t­ed drift­ers, in an email. 

The es­ti­mates in the new pa­per put the two bod­ies so far apart that their mu­t­ual grav­i­ta­tion­al pull would be a fee­ble 1/4,000 of that be­tween the Sun and Earth. The authors, José An­to­nio Ca­bal­le­ro of the In­sti­tuto de As­trofisica de Ca­narias in Ten­er­ife, Spain and col­leagues, wrote that the sys­tem seems un­like­ly to sur­vive with­out be­ing yanked apart by com­pet­ing grav­i­ta­tion­al forc­es near­by. 

But this is what makes the find in­tri­g­uing, Ca­bal­le­ro wrote in an email: it might re­pre­sent a pla­net’s for­ma­tive mo­ments, “just be­fore be­ing ejected from the plan­e­tar­y sys­tem. Very prob­a­bly man­y of the iso­lat­ed plan­e­tar­y-mass ob­jects that we find form in the same way.”

Researchers call the meander­ing worlds “iso­la­ted pla­ne­tary mass ob­jects” to ac­count for the pos­si­bi­li­ty that they dif­fer fund­a­men­t­al­ly from fa­mi­liar pla­nets. More than 100 of them have been re­ported—which al­so chal­lenges plan­et-for­ma­tion the­o­ries, since plan­ets are thought to form from dusty clouds sur­round­ing their star.

Ca­bal­le­ro’s team studied an object known as S Ori 68, list­ed as pos­si­bly be­long­ing to the new breed. It in­ha­bits a clus­ter of young stars called Sig­ma Or­i­o­nis, 1,200 light-years away in the con­stel­la­tion Ori­on. A light-year is the dis­tance light trav­els in a year.

S Ori 68, as massive as about five Ju­pi­ters, seems to be close enough to a brown dwarf star to or­bit it, Ca­bal­le­ro’s team wrote. A brown dwarf is a “failed” star that is too light to ig­nite nu­cle­ar fu­sion, the proc­ess that pow­ers nor­mal stars. This brown dwarf, called SE 70, turned up in re­cent sky sur­veys, they added; it seems to weigh as much as about 45 Ju­pi­ters and to lie around 1,700 times fur­ther from the plan­et as our Sun is from us. 

This would make the sys­tem at least around 56 times wid­er than our So­lar Sys­tem as meas­ured from the Sun to Nep­tune, now of­fi­cial­ly the fur­thest plan­et. The new­found sys­tem would al­so be around ei­ther se­ven or 20 times wid­er than the vastest such sys­tems known. (The un­cer­tain­ty is be­cause one of those is­n’t def­i­nite­ly known to be a plan­e­tar­y sys­tem.)

Ca­bal­le­ro calls the pla­net-dwarf pair a “plan­e­tar­y sys­tem” rath­er than so­lar sys­tem be­cause “so­lar” im­plies a nor­mal star, like the Sun. 

His team stud­ied the brown dwarf us­ing a spec­tro­graph, a de­vice to mea­sure the spec­t­rum of star­light, on the Wil­liam Her­schel Tel­e­scope on La Pal­ma, Spain. It will take super-precise mea­sure­ments of the two bod­ies’ mo­tions to as­cer­tain that they’re or­biting each oth­er, Ca­bal­le­ro and col­leagues wrote, but this task may have to await the next gen­er­a­tion of tele­scopes to be­come prac­ti­cal.


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Strange new objects that redefine the term “planet” are constantly coming to light. But even astronomers used to this got a jolt six years ago, with the announced discovery of apparent orphan planets. These seem to roam the cosmos aimlessly, with no star to pin them into a set orbit. Now, some astronomers claim that at least one of these objects may not be a free-floater after all. It might instead just be on a gravitational leash so long, it’s part of by far the vastest known planetary system, more than 50 times wider than ours. Not everyone is convinced. Nonetheless, at a time when some scientists have begun exuberantly suggesting that the cosmos is buzzing with unchained worlds, the researchers in the new study say it may be time to step back and reconsider our concept of a solar system. If the findings prove correct, “not all ‘isolated’ planetary-mass objects are really isolated,” wrote Jose Antonio Caballero of the Instituto de Astrofisica de Canarias in Tenerife, Spain and colleagues in a paper describing their findings. The work is to appear in the research journal Astronomy & Astrophysics. But they admit they have shown only that a candidate planet and its star are close enough to orbit each other, not that they do. It’s “pretty speculative,” wrote Phil Lucas, an astronomer at the University of Hertfordshire in Hatfield, U.K.—a co-discoverer of the first reported free-floaters—in an email. If Caballero’s estimates are accurate, the orbiting bodies would be so far apart that their gravitational attraction on each other would be markedly feeble, roughly 1/4,000 of that between the Sun and Earth. “It seems dubious that the system can survive” without being torn apart by competing gravitational forces nearby, they wrote. But this is what makes the find interesting, Caballero added in an email: his team might have captured “the very moment of the formation of an isolated planetary-mass object, just before being ejected from the planetary system. Very probably many of the isolated planetary-mass objects that we find form in the same way.” Caballero’s team examined an object known as S Ori 68, formerly listed as a possible free-roaming planet, in a young star cluster called sigma Orionis, 1,200 light-years from Earth in the constellation Orion. A light-year is the distance light travels in a year. Researchers announced the sighting of the apparent free-roamer in 2000, and over 100 have been reported since then. These findings also challenged planet-formation theories, because planets are thought to form from gas and dust surrounding their parent star. S Ori 68, which weighs about five times as much as Jupiter, seems to be close enough to a brown dwarf star to orbit it, Caballero’s team wrote. A brown dwarf is a “failed” star that is too light to ignite nuclear fusion, the process that powers normal stars. This brown dwarf, known as SE 70, came to light in stellar surveys in the past few years, Caballero wrote. He calls the supposedly orbiting bodies a “planetary system” rather than solar system because “solar” implies a normal star, like the Sun. His team estimates that the brown dwarf to weigh as much as 45 Jupiters, or 1/23 the weight of our Sun, and to be around 1,700 times farther from the planet as our Sun is from us. This would also make the new “planetary system” at least around 56 times wider than our Solar System as measured from the Sun out to Neptune, now officially the furthest planet. The newly described system would also be around either seven or 20 times wider than the vastest such systems known. (The uncertainty is because one of those isn’t definitely known to be a planetary system.) The astronomers studied the brown dwarf using a spectrograph on the William Herschel Telescope on La Palma, Spain. A spectrograph breaks up light into its constituent parts and records the resulting spectrum. Super-accurate measurements of the two bodies’ motions will be needed to ascertain that they are really orbiting each other, Caballero and colleagues wrote, but this task may have to await the next generation of telescopes to become practical.