From Stars to Planets: The Loners

Brown dwarf glows feebly in the dark depths of space. Artwork by David Aguilar, Harvard-Smithsonian CFA

The word plan­et means “wan­der­er”; the Greeks called them planētēs aster, “wan­der­ing stars”. Indeed plan­ets trav­el on their orbits, wan­der­ing around their stars for mil­lions and bil­lions of years. Stars move along their paths in galax­ies, wan­der­ing, until the end of their time. Some objects, how­ev­er, do not fol­low this famil­iar pat­tern. These objects include plan­ets, brown dwarfs, stars, and even black holes, bring­ing up the most chal­leng­ing set­ting for a sci­ence fic­tion sto­ry.

Shad­owy neigh­bors

Our Galaxy alone is full of unusu­al stuff. Objects known as brown dwarfs were only a the­o­ret­i­cal con­cept until they were first dis­cov­ered in 1995. It is now argued that there might be as many brown dwarfs as there are stars, and our clos­est neigh­bor might turn out to be an ultra­cool brown dwarf rather than Prox­i­ma Cen­tau­ri.

Brown dwarf glows feebly in the dark depths of space. Artwork by David Aguilar, Harvard-Smithsonian CFA
Brown dwarf glows fee­bly in the dark depths of space. Art­work by David Aguilar, Har­vard-Smith­son­ian CFA

These objects fall some­where between the small­est stars and the largest plan­ets on the heav­en­ly spec­trum, and have mass­es that range between twice the mass of Jupiter and the low­er mass lim­it for nuclear reac­tions (0.08 solar mass­es). They are thought to form the same way low-mass stars do — from a col­laps­ing cloud of gas and dust. How­ev­er, as the cloud col­laps­es, it does not form an object which is dense enough at its core to trig­ger and main­tain hydro­gen-burn­ing nuclear fusion reac­tions. Their spec­tral char­ac­ter­is­tics are dif­fer­ent to those of very cool stars, show­ing an absorp­tion line of the short-lived ele­ment lithi­um. Fur­ther­more, they have ful­ly con­vec­tive sur­faces and inte­ri­ors, with no chem­i­cal dif­fer­en­ti­a­tion by depth. In young brown dwarfs, when com­bined with rapid rota­tion, their tur­bu­lent inte­ri­or motion can lead to a tan­gled mag­net­ic field that can heat their upper atmos­pheres, or coro­nas, to a few mil­lion degrees Cel­sius. Even if not fusion pow­ered, these mys­te­ri­ous objects are also known to have flares.

The nuclear fusion is what fuels a star and caus­es it to shine, but brown dwarfs are very cool and dim com­pared to stars. The coolest brown dwarf yet dis­cov­ered is about as hot as boil­ing water, with a tem­per­a­ture of less than 100 degrees Cel­sius (~ 370 K).

Update Sep­tem­ber 7th 2013: A new study shows that some brown dwarf stars may be as cool as room tem­per­a­ture.

Dwarf comparison: from stars to planets
Dwarf com­par­i­son: from stars to plan­ets

The trou­ble with brown dwarfs is that they are hard to find. An iso­lat­ed (not in a mul­ti­ple sys­tem) brown dwarf is typ­i­cal­ly vis­i­ble only at ages < 1 Gyr because of the rapid­ly fad­ing lumi­nos­i­ty. The lighter brown dwarfs are more sen­si­tive to this effect. Young brown dwarfs are vis­i­ble at rel­a­tive­ly large dis­tances but evolve rapid­ly, mak­ing it dif­fi­cult to catch them when they are in their ear­li­est stages of for­ma­tion. Old brown dwarfs will only be vis­i­ble if they are near­by. So we have more chance of dis­cov­er­ing brown dwarfs that have just recent­ly formed.

Dump­ing more mass on a brown dwarf doesn’t make it big­ger, it just makes it denser. A 70-Jupiter-mass and 20-Jupiter-mass brown dwarf are both about the size of Jupiter.

From up close, a young brown dwarf would look like a low-mass star, but an old brown dwarf would look more like Jupiter.

Brown dwarfs aren’t brown, they would look red to the naked eye.

Brown dwarfs radi­ate most of their ener­gy in infrared light.

Some brown dwarfs spin so fast that they com­plete one rota­tion in less than an hour.

Brown dwarfs have hydro­gen cores.

The aver­age den­si­ty of a brown dwarf is about 70 grams per cubic cen­time­ter, which is 5 times the den­si­ty at the cen­ter of the Earth.

- Robert Naeye, Astron­o­my mag­a­zine, August 1999, p. 36–42

Brown dwarfs have also been dis­cov­ered embed­ded in large clouds of gas and dust. Accre­tion discs were detect­ed around some of these failed stars, even around those as small as 10 Jupiter mass­es. Astronomers dis­cov­ered that some disks con­tain dust par­ti­cles that have crys­tal­lized and are stick­ing togeth­er in what may be the ear­ly phas­es of plan­et assem­bling. A rel­a­tive­ly large and many small grains of min­er­al called olivine was found; anoth­er sign of dust gath­er­ing up into plan­ets is the flat­ten­ing of brown dwarfs’ disks. But such sys­tems will be tiny com­pared to our own Solar Sys­tem.

Brown dwarfs emit faint vis­i­ble light, but are cool enough to retain a some­what Jupiter-con­sis­tent atmos­phere. For exam­ple, Gliese 229B, dis­cov­ered in 1995. Its lumi­nos­i­ty is about one tenth of the faintest star and its spec­trum has large amounts of methane and water vapor. Methane could not exist if the sur­face tem­per­a­ture were above 1200K. Astronomers con­sid­er its tem­per­a­ture to be about 950K (com­pared to Jupiter’s 130K), its mass to be between 0.02 and 0.05 of solar, and the age of the bina­ry sys­tem to be between 2 and 4 bil­lion years. It has a smog­gy haze lay­er deep in its atmos­phere, essen­tial­ly mak­ing it, much fainter in vis­i­ble light than it would oth­er­wise be. It is pos­si­ble that ultra­vi­o­let light from its com­pan­ion star changes its atmos­pher­ic prop­er­ties from those of an iso­lat­ed brown dwarf. The­o­ry now also sug­gests that young brown dwarfs are hot enough (with tem­per­a­tures as high as 2000 Kelvin) to have clouds of iron and sil­i­cates. These clouds “rain out” as the brown dwarf cools and becomes dim­mer. But this rain­ing out caus­es a tem­po­rary bright­en­ing as obscur­ing clouds are cleared from the atmos­phere. Con­sid­er­ing the age and the tem­per­a­ture of Gliese 229b, its atmos­phere should be clear of clouds, leav­ing it a fea­ture­less ball glow­ing a dull red, like a coal, from inter­nal heat. Any moons that might had formed too close to the brown dwarf would have been torn apart by tidal forces long ago. Sur­viv­ing moons, if they exist, may be heat­ed enough by tidal forces for methane or nitro­gen gey­sers to form.

This paper describes a col­lec­tion of evo­lu­tion­ary mod­els for brown dwarfs and very-low-mass stars for dif­fer­ent atmos­pher­ic metal­lic­i­ties, with and with­out clouds.

Children of the Demon Planet. The massive Brown Dwarf with its array of moons, by Christian Thrower*
Chil­dren of the Demon Plan­et. The mas­sive Brown Dwarf with its array of moons, by Chris­t­ian Throw­er*

*You should check out his web­site, Christian’s paint­ings are awe­some!

Brown dwarfs are mem­bers of a group known as sub­stel­lar objects. This group also includes plan­e­tary mass objects, or planemos, rang­ing from satel­lite plan­ets and belt plan­ets to rogue plan­ets, to sub-brown dwarfs.

All plan­ets are plan­e­tary mass objects by def­i­n­i­tion; their mass is expect­ed to be greater than of minor objects (e.g. mete­oroids, aster­oids or minor plan­ets) but small­er than that of brown dwarfs or stars, yet a plan­e­tary mass object (PMO) is a celes­tial object, which do not con­form to typ­i­cal expec­ta­tions for a plan­et.

Free float­ing plan­ets not orbit­ing a star may be rogue plan­ets eject­ed from their sys­tems (dur­ing sys­tem for­ma­tion, death or some insta­bil­i­ty with­in its life­time), or objects that have formed through cloud-col­lapse rather than accre­tion. Iso­lat­ed PMOs with mass­es low­er than the 13-Jupiter-mass def­i­n­i­tion of a brown dwarf, which were not eject­ed, but have always been free-float­ing and are thought to have formed in a sim­i­lar way to stars, are called sub-brown dwarfs.

A rogue plan­et (also known as an inter­stel­lar plan­et, or orphan plan­et) is a plan­e­tary-mass object that has been eject­ed from its sys­tem and is no longer grav­i­ta­tion­al­ly bound to any star, brown dwarf or oth­er such object, and that there­fore orbits the galaxy direct­ly. Recent­ly, some astronomers have esti­mat­ed that there may be twice as many Jupiter-sized rogue plan­ets as there are stars. Is it get­ting crowd­ed here?

If inter­stel­lar space is full of sub­stel­lar objects, they might become our step­ping stones for robot­ic (or even manned) mis­sions in the (very) dis­tant future. It would mean inter­stel­lar space could be reached through an “island-hop­ping” strat­e­gy. Even if there won’t be much of plan­e­tary sys­tems, the resources could still be used by probes (or humans, to live in ships or what­ev­er space habi­tats there might be devel­oped).

Plan­ets and sub­stel­lar objects aside, there are even more bizarre things hap­pen­ing out there.

Stel­lar ham­burg­ers

Blue strag­glers are main sequence stars in open or glob­u­lar clus­ters that are more lumi­nous and bluer than stars at the main sequence turn-off point for the clus­ter. With mass­es two to three times that of the rest of the main sequence clus­ter stars, blue strag­glers seem to be excep­tions to the rule of posi­tion­ing all clus­ter stars on a clear­ly defined curve set by the age of the clus­ter, with the posi­tions of indi­vid­ual stars on that curve deter­mined sole­ly by their ini­tial mass.

The expla­na­tion for this might lie in col­li­sions and mass trans­fer between bina­ry stars of the clus­ter. The merg­er of two stars would cre­ate a sin­gle more mas­sive star, poten­tial­ly with a mass larg­er than that of stars at the main sequence turn-off point. These blue stag­glers are com­mon res­i­dents of the galax­ies. How­ev­er, some blue stag­glers have even more dra­mat­ic emer­gence.

Stud­ies have already shown that our Galaxy is able to “eject” stars once in about every 100,000 years. And the one respon­si­ble for such deeds is none oth­er than the black hole at the cen­ter of the Galaxy.

The tragic case of the star HE 0437-5439. Credit: NASA, ESA, E. Feild (STScI)
The trag­ic case of the star HE 0437–5439. Cred­it: NASA, ESA, E. Feild (STScI)

The incred­i­ble fate of HE 0437–539 sum­ma­rized in 5 steps. 1: a triple-star sys­tem was drawn to the black hole in the cen­ter of our Galaxy. 2: one of the three stars was grabbed by the black hole and the two oth­ers eject­ed. 3: the duo broke away from our Galaxy. 4: on aging, the two stars merged. 5: the merg­er gave rise to a blue strag­gler which con­tin­ues to move away from our Galaxy. Alone… Well, maybe not.

There will be only one…

The sim­i­lar sit­u­a­tion seems to be with anoth­er objects galax­ies might like to dis­pose of. They indeed are very vio­lent crea­tures, and dur­ing their “mat­ing rit­u­als” they prob­a­bly can even rip out hearts.

This artist's conception shows a rogue black hole that has been kicked out from the center of two merging galaxies. The black hole is surrounded by a cluster of stars that were ripped from the galaxies. Credit: STScI
This artist’s con­cep­tion shows a rogue black hole that has been kicked out from the cen­ter of two merg­ing galax­ies. The black hole is sur­round­ed by a clus­ter of stars that were ripped from the galax­ies. Cred­it: STScI

In such hyper­com­pact stel­lar sys­tems the super­mas­sive black hole keeps the stars mov­ing in very tight orbits about the cen­ter of the clus­ter, where it resides. These objects are believed to be fair­ly com­mon. In the­o­ry, hun­dreds of mas­sive black holes left over from the age of galaxy for­ma­tion could be lurk­ing in the near­by uni­verse, because they are expect­ed to be grav­i­ta­tion­al­ly bound to the galaxy clus­ter that had pro­duced them. So the best place for find­ing such objects would be regions of space dense with thou­sands of galax­ies that have been merg­ing for a long time, since black hole merg­ers with­in these galax­ies may have result­ed in vio­lent kicks.

Update: April 2013

New com­put­er sim­u­la­tions pre­dict as many as 2000 black holes liv­ing on the out­skirts of the Milky Way. Some might have been stripped bare, while oth­ers may car­ry a few clus­ters of stars and dark mat­ter.

Jeno Marz
JENO MARZ is a science fiction writer from Latvia, Northern Europe, with background in electronics engineering and computer science. She is the author of two serial novels, Falaha’s Journey: A Spacegirl’s Account in Three Movements and Falaha’s Journey into Pleasure. Marz is current at work on a new SF trilogy. All her fiction is aimed at an adult audience.

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