The seven worlds of 75$33,67

All are Earth sized and some might have wa­ter

Sky at Night Magazine - - SPACE IN 2017 -

A new plan­e­tary sys­tem has dom­i­nated the field of ex­o­plan­e­tary science this year: TRAPPIST-1. In May, a team of astronomer­s an­nounced they had found a sys­tem of seven Earth-sized plan­ets in or­bit around an ul­tra-cool red dwarf, des­ig­nated TRAPPIST-1 af­ter the small tele­scope in Chile that it was dis­cov­ered with. The plan­ets are all close to the star, with or­bits last­ing a mat­ter of days, but the star’s cool tem­per­a­ture means that some could still po­ten­tially hold liq­uid wa­ter on their sur­faces.

The sys­tem was found dur­ing TRAPPIST’s sur­vey of 50 low-mass stars, a pro­to­type search to test the idea that such stars could be home to ob­serv­able plan­ets. Smaller, dim­mer stars are well suited to study­ing tran­sit­ing plan­ets be­cause the signs of a planet aren’t as swamped by the star’s glare.

“We are al­ready do­ing many fol­low up ob­ser­va­tions,” says Michael Gil­lon from the Univer­sité de Liège, who led the TRAPPIST team. “Most notably we are try­ing to mea­sure the masses of the plan­ets.”

The masses range be­tween 0.2 and 1.6 times the mass of the Earth, and once these are pinned down pre­cisely re­searchers will be able to mea­sure their den­sity. In turn, this will give them an idea of the plan­e­tary com­po­si­tion and what kind of at­mos­phere they might have.

“We are cur­rently try­ing to look for at­mos­pheres with Hubble but it can only find low-den­sity at­mos­pheres that are rich in hy­dro­gen. But, if we can de­tect hy­dro­gen, that’s a strong in­di­ca­tion that they have an at­mos­phere that is rich in wa­ter,” says Gil­lon.

Though astronomer­s will likely have to wait un­til the next gen­er­a­tion of tele­scopes are op­er­a­tional to re­ally un­der­stand these plan­ets, they are al­ready try­ing to work out what they might be able to find.

“There was a study of the star it­self,” Gil­lon con­tin­ues. “Its ul­tra­vi­o­let and X-ray emis­sion isn’t in­tense enough to have stripped the plan­ets of their at­mos­phere and wa­ter com­pletely, so they re­main good hab­it­able planet can­di­dates.” The emis­sions also show that the star, and so the plan­ets, are be­tween 5 to 10 bil­lion years old, much older than the Earth’s 4.5 bil­lion years. Could life have evolved on these plan­ets?

“The con­di­tions are not Earth-like, it’s not Earth Two. But at least a few of these plan­ets could have liq­uid wa­ter. That, along with the en­ergy from the star and the fact that or­ganic mol­e­cules com­posed of hy­dro­car­bons are quite com­mon, means you could have the emer­gence of life on one of these plan­ets,” he says.

The seven plan­ets all or­bit close to their par­ent star, an ul­tra-cool red dwarf

De­spite their prox­im­ity to their star, sev­eral of the TRAPPIST worlds could pos­sess liq­uid wa­ter

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