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Radio glow could reveal elusive planets, study finds

April 19, 2011
Courtesy of the Royal Astronomical Society
and World Science staff

Ra­di­o wave emis­sions simi­lar to Earth’s North­ern Lights could re­veal the pres­ence of oth­er­wise hard-to-find plan­ets out­side our so­lar sys­tem, a stud­y sug­gests.

Al­though tech­niques have im­proved for de­tect­ing such dis­tant worlds, called exo­plan­ets, it re­mains a chal­lenge to find those that or­bit their suns at large dis­tances. Now, a sci­entist has cal­cu­lat­ed that emis­sions from plan­ets like Ju­pi­ter should be de­tect­a­ble by ra­di­o tele­scopes such as the Netherlands-based Low Fre­quen­cy Ar­ray, to be com­plet­ed lat­er this year.

An image mapping ultra­violet light em­is­sions from an aur­ora on Saturn. (Cour­tesy NA­SA)


The i­de­a is that gi­ant gas plan­ets would re­veal them­selves through ra­dio emis­sions that are part of a phe­nom­e­non called au­ro­ras, al­so known to oc­cur on Ju­pi­ter and Sat­urn. 

Such planets are of in­ter­est to astro­n­om­ers, in­clud­ing those search­ing for extra­ter­rest­rial life. Although gas gi­ants prob­a­bly can’t sup­port life them­selves, some sci­ent­ists be­lieve they may of­ten serve as good sign­posts to life be­cause near­by plan­ets, or their own moons, could be hab­it­a­ble.

Jon­a­than Nichols of the U­ni­ver­si­ty of Leices­ter, U.K., pre­sented the new re­search at the U.K. Roy­al As­tro­nom­i­cal So­ci­ety’s na­tion­al meet­ing in Llan­dud­no, Wales, on A­pril 18.

“This is the first stud­y to pre­dict the ra­di­o emis­sions by ex­o­plan­e­tary sys­tems,” he sa­id. At Ju­pi­ter and Sat­urn, “we see ra­di­o waves as­so­ci­at­ed with au­ro­ras gen­er­at­ed by in­ter­ac­tions with i­on­ized [e­lec­tri­cal­ly charged] gas es­cap­ing from the vol­can­ic moons, Io and En­cel­a­dus. Our stud­y shows that we could de­tect emis­sions from ra­di­o au­ro­ras from Ju­pi­ter-like sys­tems or­biting at dis­tances as far out as Plu­to.”

Au­ro­ras are light dis­plays in the skies caused by charged par­ti­cles stream­ing a­long the mag­net­ic field lines of a plan­et in­to its at­mos­phere. On Earth, Par­ti­cles from the Sun br­ing a­bout au­ro­ras, al­so called North­ern Lights or South­ern Lights ow­ing to their ap­pear­ance near the poles. On Ju­pi­ter and Sat­urn, par­ti­cles from their own moons are al­so im­pli­cat­ed in au­ro­ral dis­plays, which may in­clude ra­dio waves as these are simp­ly a low-energy form of light.

The con­nect­ion with exo­plan­et detect­ion arises be­cause most known exo­plan­ets have been found by the so-called trans­it meth­od, which de­tects a dim­ming in light as a plan­et moves in front of a star, or by look­ing for a wob­ble as a star is tugged by the grav­i­ty of an or­biting plan­et. With both tech­niques, it’s eas­i­est to de­tect plan­ets close in to the star and mov­ing quick­ly. Of the hun­dreds of exoplan­ets found so far, less than a tenth or­bit their stars at dis­tances like those of the plan­ets in our sys­tem to the Sun, Nichols sa­id.

“Ju­pi­ter and Sat­urn take 12 and 30 years re­spec­tive­ly to or­bit the Sun, so you would have to be in­cred­i­bly luck­y or look for a ver­y long time to spot them by a trans­it or a wob­ble,” Nichols ex­plained.

Ra­di­o au­ro­ras could give a­way these plan­ets to a dis­tant ob­serv­er, he ar­gued.

He ex­am­ined how the ra­di­o emis­sions for Ju­pi­ter-like exo­plan­ets would be af­fect­ed by the plan­et’s spin rate, the rate of i­on­ized gas out­flow from a moon, the or­bital dis­tance of the plan­et and the bright­ness of the par­ent star in ultra­violet light. He found that, in man­y sce­nar­i­os, exo­plan­ets or­biting ultra­violet-bright stars at be­tween one and 50 times the Earth-Sun dis­tance would gen­er­ate e­nough ra­di­o pow­er to be de­tect­a­ble from Earth. For the bright­est stars and fastest spin­ning plan­ets, the emis­sions would be de­tect­a­ble from sys­tems 150 light years a­way, Nichols main­tains. A light year is the dis­tance light trav­els in a year.

“In our So­lar Sys­tem, we have a sta­ble sys­tem with out­er gas gi­ants and in­ner ter­res­tri­al plan­ets, like Earth, where life has been a­ble to evolve,” he sa­id. “Be­ing a­ble to de­tect Ju­pi­ter-like plan­ets may help us find plan­etary sys­tems like our own, with oth­er plan­ets that are capa­ble of sup­porting life.”


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Radio wave emissions of a type related to Earth’s Northern Lights could reveal the presence of otherwise hard-to-find planets outside our solar system, a study suggests. Although techniques have improved for detecting such distant worlds, called exoplanets, it remains a challenge to find those that orbit their suns at large distances. Now, a scientist has calculated that emissions from planets like Jupiter should be detectable by radio telescopes such as the Netherlands-based Low Frequency Array, to be completed later this year. The idea is that giant gas planets would reveal themselves through a phenomenon called radio auroras, also known to occur on Jupiter and Saturn. Although gas giants probably can’t support life themselves, some astronomers believe they may often serve as good signposts to extraterrestrial life because nearby planets, or their own moons, could be habitable. Jonathan Nichols of the University of Leicester, U.K., presented the new research at the U.K. Royal Astronomical Society’s national meeting in Llandudno, Wales, on April 18. "This is the first study to predict the radio emissions by exoplanetary systems,” he said. At Jupiter and Saturn, “we see radio waves associated with auroras generated by interactions with ionized [electrically charged] gas escaping from the volcanic moons, Io and Enceladus. Our study shows that we could detect emissions from radio auroras from Jupiter-like systems orbiting at distances as far out as Pluto.” Generally speaking, auroras are light displays in the skies caused by charged particles streaming along the magnetic field lines of a planet into its atmosphere. On Earth, Particles from the Sun bring about auroras, also called Northern Lights or Southern Lights owing to their appearance near the poles. On Jupiter and Saturn, particles from their own moons are also implicated in auroral displays. That’s where the subject of exoplanets comes in. Most exoplanets have been detected by the so-called transit method, which detects a dimming in light as a planet moves in front of a star, or by looking for a wobble as a star is tugged by the gravity of an orbiting planet. With both techniques, it’s easiest to detect planets close in to the star and moving quickly. Of the hundreds of exoplanets found so far, less than a tenth orbit their stars at distances like those of the planets in our system to the Sun, Nichols said. “Jupiter and Saturn take 12 and 30 years respectively to orbit the Sun, so you would have to be incredibly lucky or look for a very long time to spot them by a transit or a wobble,” Nichols explained. Radio auroras could give away these planets to a distant observer, he argued. He examined how the radio emissions for Jupiter-like exoplanets would be affected by the planet’s spin rate, the rate of ionized gas outflow from a moon, the orbital distance of the planet and the brightness of the parent star in ultraviolet light. He found that, in many scenarios, exoplanets orbiting ultraviolet-bright stars at between one and 50 times the Earth-Sun distance would generate enough radio power to be detectable from Earth. For the brightest stars and fastest spinning planets, the emissions would be detectable from systems 150 light years away, Nichols maintains. A light year is the distance light travels in a year. "In our Solar System, we have a stable system with outer gas giants and inner terrestrial planets, like Earth, where life has been able to evolve,” he said. “Being able to detect Jupiter-like planets may help us find planetary systems like our own, with other planets that are capable of supporting life.” n