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The sys­tem of plan­ets found or­bit­ing nearby dwarf star TRAP­PIST-1 OC[ DG QWT DGUV EJCPEG [GV QH ƂPFKPI CNKGP NKHG

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TRAP­PIST-1, an ul­tra­cool dwarf star lo­cated just 40 lightyears from Earth in the Aquar­ius con­stel­la­tion, was first de­tected by re­searchers from Liege us­ing the Tran­sit­ing Plan­ets and Plan­etes­i­mals Small Tele­scope (TRAP­PIST) in Chile, and later con­firmed by NASA’s Spitzer Space Tele­scope and the Very Large Tele­scope, also in Chile.

The plan­ets were de­tected by ob­serv­ing dips in the star’s light out­put caused by each of the seven plan­ets pass­ing in front of it, events known as tran­sits.

The re­searchers found that all of the plan­ets are com­pa­ra­ble in size to the Earth, while den­sity mea­sure­ments sug­gest that the in­ner­most six are rocky.

Cur­rent cli­mate mod­els sug­gest the three in­ner­most plan­ets are prob­a­bly too hot to sup­port liq­uid water, and the one fur­thest from the star is too cold. How­ever, the re­main­ing three sit com­fort­ably within the hab­it­able zone and could host oceans of sur­face water – a fea­ture thought to be es­sen­tial for the ex­is­tence of life.

“The en­ergy out­put from dwarf stars like TRAP­PIST-1 is much weaker than that of our Sun. Plan­ets would need to be in far closer or­bits than we see in the So­lar Sys­tem if there is to be sur­face water,” said re­searcher Dr Amaury Tri­aud. “For­tu­nately, it seems that this kind of com­pact con­fig­u­ra­tion is just what we see around TRAP­PIST-1.”

The star is rel­a­tively small, just 8 per cent the mass of the Sun, and would ap­pear to glow sal­mon pink when ob­served from the sur­face of the plan­ets, the re­searchers say.

Now that as­tronomers know that the plan­ets are there, the next job is to find out what they are re­ally like. The first step is to make an ac­cu­rate de­ter­mi­na­tion of their den­si­ties. When search­ing for hab­it­able worlds, rocky plan­ets are the clear pref­er­ence be­cause – put sim­ply – they pro­vide a sur­face for life forms to walk, slither or other­wise move across.

The Euro­pean Space Agency (ESA) will launch CHEOPS (CHar­ac­ter­is­ing ExOPlanet Satel­lite) in 2018. The main sci­ence goals of the mis­sion are to mea­sure the den­si­ties of plan­ets with radii be­tween one and six times of Earth. The TRAP­PIST-1 sys­tem YKNN DG JKIJ QP VJG NKUV |

The next step will be to an­a­lyse the plan­ets’ at­mos­pheres to see if any look like they could be hab­it­able. “The main goal will be try­ing to de­tect the sig­na­ture of water,” said CHEOPS sci­en­tist Dr Vin­cent Bour­rier.

Water vapour in a planet’s at­mos­phere could be­tray wide­spread oceans and a water cy­cle. Its sig­na­ture ap­pears in the in­frared re­gion of the spec­trum and this is where the NASA-built James Webb Space Tele­scope (JWST) comes in.

ESA will launch the JWST in the same year as CHEOPS. With its 6.5m-di­am­e­ter in­frared mir­ror, JWST will make analysing exoplanet at­mos­pheres eas­ier than ever. One of its first tar­gets is likely to be the seven worlds of the TRAP­PIST-1 so­lar sys­tem.

While find­ing water vapour would in­crease the be­lief that the planet un­der in­ves­ti­ga­tion is po­ten­tially hab­it­able, there are other fac­tors that could af­fect a planet’s abil­ity to sup­port life.

To in­ves­ti­gate those, Bour­rier and his team have al­ready used the Hub­ble Space Tele­scope to look at the ul­tra­vi­o­let sig­na­ture of TRAP­PIST-1’s two in­ner­most plan­ets. His work shows that those plan­ets could have had their at­mos­pheres com­pletely eroded away by the ra­di­a­tion from the star – ren­der­ing the plan­ets bar­ren. Could this have hap­pened to the other worlds of TRAP­PIST-1?

Ac­tu­ally prov­ing that a planet is hab­it­able may be re­ally tough. As­tronomers will have to look for ‘biomark­ers’. These are gases that only ex­ist to­gether in an at­mos­phere be­cause they are be­ing re­plen­ished by the me­tab­o­lisms of liv­ing crea­tures. Oxy­gen and meth­ane are good ex­am­ples in our own at­mos­phere. So far, there are no firm plans to build a space tele­scope ca­pa­ble of mak­ing such an ex­act­ing mea­sure­ment, although NASA and ESA have both stud­ied en­gi­neer­ing con­cepts.

NASA’s next exoplanet mis­sion, the Tran­sit­ing Exoplanet Sur­vey Satel­lite (TESS), also launch­ing next year, could re­veal many more so­lar sys­tems like TRAP­PIST-1. This mis­sion will sur­vey 200,000 stars and is ex­pected to dis­cover thou­sands of ex­o­plan­ets, from the size of Earth up to Jupiter and larger.”


Stu­art Clark is an as­tron­omy writer. His most re­cent book is the The Search For Earth’s Twin (£20, Quer­cus).

EX­PERT COM­MENT As the plan­ets in the TRAP­PIST-1 sys­tem are so close to­gether, they’d be vis­i­ble in each other’s skies, as seen in this il­lus­tra­tion

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