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September 16, 2010

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Planets found sharing strange dances

July 29, 2010
Courtesy of Caltech
and World Science staff

Most plan­ets or­bit in a sol­i­tary sort of maj­es­ty around their host star, too far from oth­er plan­ets in the sys­tem to be af­fected by their gra­vity.

But now, re­search­ers have found two plan­e­tary sys­tems with fea­tur­ing pairs of gas gi­ant plan­ets locked in an or­bital dance.

Hun­dreds of plan­ets have been found out­side our own so­lar sys­tem in the past 15 years. One in three of these ap­pear to have multiple plan­ets, which gen­er­al­ly seem to come in bunch­es. In just a hand­ful of cases, plan­ets have been found near enough to one anoth­er to in­ter­act gravita­t­ion­ally.

In one newfound sys­tem — two worlds or­biting the mas­sive, dy­ing star HD 200964, some 223 light-years from Earth — this dance is clos­er and tighter than any be­fore seen, said as­tron­o­mer John A. John­son of the Cal­i­for­nia In­sti­tute of Tech­nol­o­gy, one of the re­search­ers on the proj­ect.

“This new plan­et pair came in an un­ex­pected pack­age,” says John­son.

“A plan­e­tary sys­tem with such closely spaced gi­ant plan­ets would be de­stroyed quickly if the plan­ets weren’t do­ing such a well syn­chro­nized dance. This makes it a real puz­zle how the plan­ets could have found their rhyth­m,” added Er­ic Ford of the Uni­vers­ity of Flor­i­da in Gaines­ville.

A pa­per on the find­ings by John­son, Ford, and their col­la­bo­ra­tors has been ac­cept­ed for pub­lica­t­ion in the As­tro­nom­i­cal Jour­nal.

All four new­found worlds are gas gi­ants some­what like Ju­pi­ter, but heav­i­er, and were dis­cov­ered by a com­mon meth­od of meas­ur­ing the wob­ble of their par­ent stars as the plan­ets or­bit around them. 

The mem­bers of each pair are re­markably close to one anoth­er, the sci­en­tists said. The dis­tance be­tween the plan­ets or­biting HD 200964 oc­ca­sion­ally drops to 33 mil­lion miles. That’s com­pa­ra­ble to the Earth-Mars dis­tance, which for such mas­sive plan­ets makes them nearly next-door neigh­bors. The plan­ets or­biting the sec­ond star, 24 Sex­ta­nis are about 70 mil­lion miles apart. By com­par­i­son, Ju­pi­ter and Sat­urn are nev­er less than 330 mil­lion miles apart.

The pairs tug on each oth­er with pow­er­ful gravita­t­ional forc­es. That be­tween HD 200964’s two plan­ets, for ex­am­ple, is 700 times larg­er than the pull be­tween the Earth and the Moon, which causes the tides to rise and fall.

Un­like the gas gi­ants in our own so­lar sys­tem, the new plan­ets are rel­a­tively near their stars. This makes their years last only a year or two in Earth years, rath­er than, say 12 years in Ju­pi­ter’s case.

Plan­ets of­ten move around af­ter they form, in a pro­cess known as migra­t­ion. Migra­t­ion is thought to be com­monplace — it even oc­curred to some ex­tent in our own so­lar sys­tem — but it is­n’t or­der­ly. As a plan­e­tary sys­tem forms, worlds fur­ther from the star can mi­grate faster than those clos­er in, “so plan­ets will cross paths and jos­tle each oth­er around,” John­son said. “The only way they can ‘get along’ and be­come sta­ble is if they en­ter an or­bital res­o­nance.” 

Plan­ets are in “or­bital res­o­nance” if their years – the length of time in which they cir­cle the par­ent star – are sim­ple mul­ti­ples or ra­ti­os of each oth­er, such as two to one, three to two, and so forth.

For in­stance, in a 2:1 res­o­nance, an out­er plan­et will or­bit once for eve­ry two or­bits of the in­ner plan­et; in a 3:2 res­o­nance, the out­er plan­et will or­bit twice for eve­ry three pas­ses by the in­ner plan­et, and so forth. Such res­o­nances are cre­at­ed by the gravita­t­ional in­flu­ence of plan­ets on one anoth­er.

These res­o­nances form zones of sta­bil­ity in which mi­grat­ing plan­ets tend to set­tle, John­son ex­plained. A 2:1 res­o­nance — which is the case for the plan­ets or­biting 24 Sex­ta­nis — is the most sta­ble and the most com­mon pat­tern. “Plan­ets tend to get stuck in the 2:1. It’s like a really big pot­hole,” John­son says. “But if a plan­et is mov­ing very fast it can pass over a 2:1. As it moves in clos­er [to the star], the next step is a 5:3, then a 3:2, and then a 4:3.” 

John­son and his col­leagues found that the pair of plan­ets or­biting HD 200964 is locked in a 4:3 res­o­nance. “The clos­est anal­o­gy in our so­lar sys­tem is Ti­tan and Hy­pe­ri­on, two moons of Sat­urn which al­so fol­low or­bits syn­chro­nized in a 4:3 pat­tern,” said Ford. “But the plan­ets or­biting HD 200964 in­ter­act much more strongly, since each is around 20,000 times more mas­sive than Ti­tan and Hy­pe­ri­on com­bined.” 

“This is the tight­est sys­tem that’s ev­er been dis­cov­ered,” John­son added, “and we’re at a loss to ex­plain why this hap­pened. This is the lat­est in a long line of strange disco­veries about extraso­lar [out­side our so­lar sys­tem] plan­ets… each time we think we can ex­plain them, some­thing else comes along.” 

John­son and his col­leagues found the two sys­tems us­ing da­ta from the Keck Subgi­ants Plan­et Sur­vey — a search for plan­ets around stars from 40 to 100 per­cent larg­er than our own Sun. Subgi­ants rep­re­sent a class of stars that have have run out of fu­el, caus­ing their co­re to col­lapse and their out­er en­ve­lope to swell. Subgi­ants eventually be­come red gi­ants — stars with big, puffy at­mo­spheres that pul­sate, mak­ing it hard to de­tect or­biting plan­ets.

Subgi­ants, though, have char­ac­ter­is­tics that make their plan­ets easy to find, John­son said.

“Right now, we’re mon­i­tor­ing 450 of these mas­sive stars, and we are find­ing swarms of plan­ets,” he says. “Around these stars, we are see­ing three to four times more plan­ets out to a dis­tance of about three AU — the dis­tance of our as­ter­oid belt — than we see around main se­quence stars. Stel­lar mass has a huge in­flu­ence on [the] fre­quen­cy of plan­et oc­cur­rence, be­cause the amount of raw ma­te­ri­al avail­a­ble to build plan­ets scales with the mass of the star.” 

Even­tu­al­ly, per­haps 10 or 100 mil­lion years from now, subgi­ants like HD 200964 and 24 Sex­ta­nis will be­come red gi­ants. They will swell to the point where they could en­gulf the in­ner plan­et of their danc­ing pair, and throw off their out­er at­mo­spheres, chang­ing the gravita­t­ional dy­nam­ics of their whole sys­tem. “The plan­ets will then move out, and their or­bits will be­come un­sta­ble,” John­son says. “Most likely one of the plan­ets will get flung out of the sys­tem com­plete­ly.”

* * *

Most planets orbit in a solitary sort of majesty around their host star, too far from other planets in the system to be disturbed by their gravity. But now, researchers have found two planetary systems with featuring pairs of gas giant planets locked in an orbital embrace. Hundreds of planets have been found outside our own solar system in the past 15 years. One in three of these appear to have more than one planet, which it seems come in bunches. In just a handful of cases, planets have been found near enough to one another to interact gravitationally. In one newly studied system — a planetary pair orbiting the massive, dying star HD 200964, located roughly 223 light-years from Earth — the intimate dance is closer and tighter than any previously seen, said astronomer John A. Johnson of the California Institute of Technology, one of the researchers on the project. “This new planet pair came in an unexpected package,” says Johnson. “A planetary system with such closely spaced giant planets would be destroyed quickly if the planets weren’t doing such a well synchronized dance. This makes it a real puzzle how the planets could have found their rhythm,” added Eric Ford of the University of Florida in Gainesville. A paper on the findings by Johnson, Ford, and their collaborators has been accepted for publication in the Astronomical Journal. All four newfound worlds are gas giants somewhat like Jupiter, but heavier, and were discovered by a common method of measuring the wobble of their parent stars as the planets orbit around them. The members of each pair are remarkably close to one another, the scientists said. The distance between the planets orbiting HD 200964 occasionally drops to 33 million miles. That’s comparable to the Earth-Mars distance, which for such massive planets makes them nearly next-door neighbors. The planets orbiting the second star, 24 Sextanis are about 70 million miles apart. By comparison, Jupiter and Saturn are never less than 330 million miles apart. The pairs tug on each other with powerful gravitational forces. That between HD 200964’s two planets, for example, is 700 times larger than the pull between the Earth and the Moon, which causes the tides to rise and fall. Unlike the gas giants in our own solar system, the new planets are relatively near their stars. This makes their years last only a year or two in Earth years, rather than, say 12 years in Jupiter’s case. Planets often move around after they form, in a process known as migration. Migration is thought to be commonplace — it even occurred to some extent in our own solar system — but it isn’t orderly. As a planetary system forms, worlds further from the star can migrate faster than those closer in, “so planets will cross paths and jostle each other around,” Johnson said. “The only way they can ‘get along’ and become stable is if they enter an orbital resonance.” Planets are in “orbital resonance” if their years – the length of time in which they circle the parent star – are simple multiples or ratios of each other, such as two to one, three to two, and so forth. For instance, in a 2:1 resonance, an outer planet will orbit once for every two orbits of the inner planet; in a 3:2 resonance, the outer planet will orbit twice for every three passes by the inner planet, and so forth. Such resonances are created by the gravitational influence of planets on one another. These resonances form zones of stability in which migrating planets tend to settle, Johnson explained. A 2:1 resonance — which is the case for the planets orbiting 24 Sextanis — is the most stable and the most common pattern. “Planets tend to get stuck in the 2:1. It’s like a really big pothole,” Johnson says. “But if a planet is moving very fast it can pass over a 2:1. As it moves in closer [to the star], the next step is a 5:3, then a 3:2, and then a 4:3.” Johnson and his colleagues found that the pair of planets orbiting HD 200964 is locked in a 4:3 resonance. “The closest analogy in our solar system is Titan and Hyperion, two moons of Saturn which also follow orbits synchronized in a 4:3 pattern,” said Ford. “But the planets orbiting HD 200964 interact much more strongly, since each is around 20,000 times more massive than Titan and Hyperion combined.” “This is the tightest system that’s ever been discovered,” Johnson added, “and we’re at a loss to explain why this happened. This is the latest in a long line of strange discoveries about extrasolar [outside our solar system] planets… each time we think we can explain them, something else comes along.” Johnson and his colleagues found the two systems using data from the Keck Subgiants Planet Survey — a search for planets around stars from 40 to 100 percent larger than our own Sun. Subgiants represent a class of stars that have have run out of fuel, causing their core to collapse and their outer envelope to swell. Subgiants eventually become red giants — stars with big, puffy atmospheres that pulsate, making it hard to detect orbiting planets. Subgiants, though, have characteristics that make their planets easy to find, Johnson said. “Right now, we’re monitoring 450 of these massive stars, and we are finding swarms of planets,” he says. “Around these stars, we are seeing three to four times more planets out to a distance of about three AU — the distance of our asteroid belt — than we see around main sequence stars. Stellar mass has a huge influence on frequency of planet occurrence, because the amount of raw material available to build planets scales with the mass of the star.” Eventually, perhaps 10 or 100 million years from now, subgiants like HD 200964 and 24 Sextanis will become red giants. They will swell to the point where they could engulf the inner planet of their dancing pair, and throw off their outer atmospheres, changing the gravitational dynamics of their whole system. “The planets will then move out, and their orbits will become unstable,” Johnson says. “Most likely one of the planets will get flung out of the system completely.”