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Galaxy may swarm with “nomad planets”

Feb. 27, 2012
Courtesy of Stanford University
and World Science staff

Our gal­axy may be awash in home­less plan­ets, wan­der­ing through space in­stead of or­bit­ing a star, re­search­ers say.

In fact, there may be 100,000 times more “no­mad plan­ets” in the Milky Way than stars, ac­cord­ing to a new study by re­search­ers at the Kavli In­sti­tute for Par­t­i­cle As­t­ro­phys­ics and Cos­mol­o­gy, a joint in­sti­tute of Stan­ford Uni­vers­ity and the SLAC Na­t­ional Ac­cel­er­a­tor Lab­o­r­a­to­ry.

An ar­tis­tic ren­di­tion of a no­mad ob­ject wan­der­ing the in­ter­stel­lar me­di­um. The ob­ject is in­ten­tion­al­ blur­ry to rep­re­sent un­cer­tain­ty about wheth­er it has an at­mos­phere. A no­madic ob­ject may be an icy body akin to an ob­ject found in the out­er so­lar sys­tem, a more rocky ma­te­ri­al akin to as­ter­oid or even a gas gi­ant si­m­i­lar in com­po­si­tion to the most mas­sive so­lar sys­tem plan­ets and exo­plan­ets. (Image cour­tesy of Stan­ford U.)


If ob­serva­t­ions con­firm the es­ti­mate, this new class of ce­les­tial ob­jects will af­fect cur­rent the­o­ries of plan­et forma­t­ion and could change our un­der­stand­ing of the or­i­gin and abun­dance of life.

“If any of these no­mad plan­ets are big enough to have a thick at­mos­phere, they could have trapped enough heat for bac­te­ri­al life to ex­ist,” said Lou­is Stri­gari, lead­er of the team that re­ported the re­sult in a pa­per sub­mit­ted to the Monthly No­tices of the Roy­al As­tronomical So­ci­e­ty. Al­though no­mad plan­ets don’t bask in the warmth of a star, they may gen­er­ate heat through in­ter­nal ra­di­o­ac­t­ive de­cay and geo­log­ic ac­ti­vity.

Searches over the past two dec­ades have iden­ti­fied more than 500 plan­ets out­side our so­lar sys­tem, al­most all of which or­bit stars. Last year, re­search­ers de­tected about a doz­en no­mad plan­ets, us­ing a tech­nique called gravita­t­ional mi­crolens­ing, which looks for stars whose light is mo­men­tarily re­fo­cused by the gra­vity of pass­ing plan­ets.

The re­search pro­duced ev­i­dence that roughly two no­mads ex­ist for eve­ry typ­i­cal, so-called main-sequence star in our gal­axy. The new study es­ti­mates that no­mads may be up to 50,000 times more com­mon than that.

To ar­rive at what Stri­gari him­self called “an as­tro­nom­i­cal num­ber,” the Kavli In­sti­tute team took in­to ac­count the known gravita­t­ional pull of the Milky Way gal­axy, the amount of mat­ter avail­a­ble to make such ob­jects and how that mat­ter might div­vy it­self up in­to ob­jects rang­ing from the size of Plu­to to larg­er than Ju­pi­ter. Not an easy task, con­sid­er­ing no one is quite sure how these bod­ies form. Ac­cord­ing to Stri­gari, some were probably ejected from so­lar sys­tems, but re­search in­di­cates that not all of them could have formed in that fash­ion.

“To par­a­phrase Dor­o­thy from The Wiz­ard of Oz, if cor­rect, this ex­trapola­t­ion im­plies that we are not in Kan­sas an­ymore, and in fact we nev­er were in Kan­sas,” said Al­an Boss of the Car­ne­gie In­sti­tu­tion for Sci­ence, au­thor of The Crowd­ed Uni­verse: The Search for Liv­ing Plan­ets, who was not in­volved in the re­search. “The uni­verse is rid­dled with un­seen plan­etary-mass ob­jects that we are just now able to de­tec­t.”

A good count, es­pe­cially of the smaller ob­jects, will have to wait for the next genera­t­ion of big sur­vey tele­scopes, es­pe­cially the space-based Wide-Field In­fra­red Sur­vey Tel­e­scope and the ground-based Large Syn­op­tic Sur­vey Tel­e­scope, both set to beg­in opera­t­ion in the early 2020s.

A con­firma­t­ion of the es­ti­mate could lend cre­dence to an­oth­er pos­si­bil­ity men­tioned in the pa­per – that as no­mad plan­ets roam their star­ry pas­tures, col­li­sions could scat­ter their mi­cro­bi­al flocks to seed life else­where.

“Few ar­eas of sci­ence have ex­cit­ed as much pop­u­lar and pro­fes­sion­al in­ter­est in re­cent times as the prev­a­lence of life in the uni­verse,” said co-au­thor and Kavli In­sti­tute Di­rec­tor Rog­er Bland­ford. “What is won­der­ful is that we can now start to ad­dress this ques­tion quantitatively by seek­ing more of these erst­while plan­ets and as­ter­oids wan­der­ing through in­ter­stel­lar space, and then spec­u­late about hitch­hik­ing bugs.”


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Our galaxy may be awash in homeless planets, wandering through space instead of orbiting a star. In fact, there may be 100,000 times more “nomad planets” in the Milky Way than stars, according to a new study by researchers at the Kavli Institute for Particle Astrophysics and Cosmology (KIPAC), a joint institute of Stanford University and the SLAC National Accelerator Laboratory. If observations confirm the estimate, this new class of celestial objects will affect current theories of planet formation and could change our understanding of the origin and abundance of life. “If any of these nomad planets are big enough to have a thick atmosphere, they could have trapped enough heat for bacterial life to exist,” said Louis Strigari, leader of the team that reported the result in a paper submitted to the Monthly Notices of the Royal Astronomical Society. Although nomad planets don’t bask in the warmth of a star, they may generate heat through internal radioactive decay and tectonic activity. Searches over the past two decades have identified more than 500 planets outside our solar system, almost all of which orbit stars. Last year, researchers detected about a dozen nomad planets, using a technique called gravitational microlensing, which looks for stars whose light is momentarily refocused by the gravity of passing planets. The research produced evidence that roughly two nomads exist for every typical, so-called main-sequence star in our galaxy. The new study estimates that nomads may be up to 50,000 times more common than that. To arrive at what Strigari himself called “an astronomical number,” the KIPAC team took into account the known gravitational pull of the Milky Way galaxy, the amount of matter available to make such objects and how that matter might divvy itself up into objects ranging from the size of Pluto to larger than Jupiter. Not an easy task, considering no one is quite sure how these bodies form. According to Strigari, some were probably ejected from solar systems, but research indicates that not all of them could have formed in that fashion. “To paraphrase Dorothy from The Wizard of Oz, if correct, this extrapolation implies that we are not in Kansas anymore, and in fact we never were in Kansas,” said Alan Boss of the Carnegie Institution for Science, author of The Crowded Universe: The Search for Living Planets, who was not involved in the research. “The universe is riddled with unseen planetary-mass objects that we are just now able to detect.” A good count, especially of the smaller objects, will have to wait for the next generation of big survey telescopes, especially the space-based Wide-Field Infrared Survey Telescope and the ground-based Large Synoptic Survey Telescope, both set to begin operation in the early 2020s. A confirmation of the estimate could lend credence to another possibility mentioned in the paper – that as nomad planets roam their starry pastures, collisions could scatter their microbial flocks to seed life elsewhere. “Few areas of science have excited as much popular and professional interest in recent times as the prevalence of life in the universe,” said co-author and KIPAC Director Roger Blandford. “What is wonderful is that we can now start to address this question quantitatively by seeking more of these erstwhile planets and asteroids wandering through interstellar space, and then speculate about hitchhiking bugs.”