For astronomers size mat­ters. The new gen­er­a­tion of Ex­tremely Large Telescopes will show us, for the first time, what exoplanets are re­ally like. FRED WAT­SON takes a closer look.

Cosmos - - Front Page -

STARGAZ­ING LIVE, a three-night block­buster on Australia’s ABC TV, sparked a frenzy of cit­i­zen sci­ence. The chal­lenge: find the tell-tale sig­na­tures of exoplanets in a mass of data freshly down­loaded from NASA’S Ke­pler space ob­ser­va­tory. Ke­pler’s pri­mary mis­sion has been to stare at more than 150,000 stars, in the hope of record­ing minis­cule dips in bright­ness that be­tray the pas­sage of a planet across a star’s disc. This so-called ‘tran­sit method’ is to­day’s gold stan­dard for planet-find­ing, hav­ing net­ted the vast ma­jor­ity of the 3,633 exoplanets found so far. Grey’s con­tri­bu­tion to this tally was to find a star with not one but four tran­sit­ing plan­ets.

It was the first ex­o­planet dis­cov­ery in 1995 that trig­gered the cur­rent in­dus­trial-scale pro­duc­tion line of ex­o­planet iden­ti­fi­ca­tion. A half-jupiter-sized world with the unin­spir­ing name of 51 Peg b, it was found not be­cause it dimmed the light of its par­ent star but be­cause of its mo­tion around it. Pro­fes­sional astronomers with mod­er­ately large telescopes have the where­withal to mea­sure a star’s speed very ac­cu­rately, typ­i­cally at the pace of a few me­tres per sec­ond. That is pre­cise enough to gauge a star’s to-and-fro mo­tion as it is pulled off-cen­tre by an or­bit­ing planet.

Astronomers use a de­vice known as a spec­tro­graph to re­veal the rain­bow spec­trum of light from a star. Like a colour­ful bar code, the spec­tro­graph car­ries di­ag­nos­tic in­for­ma­tion about the star. Its bands shift slightly as the star speeds up or slows down (rel­a­tive to the point of mea­sure­ment). Us­ing de­tected shifts in the bands to re­veal the pres­ence of a planet is known as the ‘Dop­pler wob­ble’ tech­nique.

In the first years of ex­o­planet dis­cov­ery it was by far the most pro­duc­tive method, so long as you had ac­cess to a te­le­scope with a spec­tro­graph. Then, in 2009, along came Ke­pler and every­thing changed. The sole mis­sion of NASA’S space te­le­scope was to search for exoplanets by iden­ti­fy­ing sud­den dips in the bright­ness of stars.

The space ob­ser­va­tory’s suc­cess spawned a new breed of ground-based ex­o­planet hunters, aided by the power and af­ford­abil­ity of new tech­nol­ogy. Us­ing in­creas­ingly sen­si­tive cam­eras com­bined with com­puter anal­y­sis, am­a­teurs could ex­ploit the tran­sit tech­nique with telescopes far smaller than those his­tor­i­cally needed.

So the pace of ex­o­planet dis­cov­ery has ex­ploded and shows no sign of slow­ing down. The large sam­ple now avail­able re­veals a di­ver­sity of plan­e­tary sys­tems that has stag­gered astronomers and shat­tered cher­ished no­tions about sys­tem for­ma­tion. We had be­lieved, for in­stance, that the line-up of our So­lar Sys­tem – with small rocky plan­ets close in and big gassy ones fur­ther out – re­flected fun­da­men­tal laws about the way so­lar sys­tems form, and our mod­els backed that up. Many of the alien sys­tems, how­ever, have gi­ant gas plan­ets within scorch­ing dis­tance of their sun. While Jupiter take 12 years to or­bit the Sun, so-called ‘hot Jupiters’ take only a few days.

These gi­ant hot gas plan­ets nes­tled close to their star were the eas­i­est to find via the Dop­pler wob­ble tech­nique, due to the de­gree they warped their star’s mo­tion. Us­ing the tran­sit method, we have also found

But own­er­ship does not in it­self dic­tate where the telescopes will go. To per­form prop­erly an ELT needs exquisite at­mo­spheric con­di­tions, and that lim­its pos­si­ble sites to a hand­ful of moun­tain-top lo­ca­tions. The GMT and Euro­pean ELT will peer into the South­ern sky from the Ata­cama desert in Chile. The TMT, which will peer into the North­ern sky, is still weigh­ing up sites on the is­lands of Mauna Kea in Hawaii or La Palma in the Ca­nary Is­lands.

Com­mon to all these ELT projects is tech­nol­ogy to re­duce the ef­fects of tur­bu­lence in the Earth’s at­mos­phere. The twin­kling of stars may in­spire po­ets but it puts a se­ri­ous damper on ob­serv­ing exoplanets. Twin­kling turns star images into in­flated wob­bling blobs of light that hide all the de­tail and re­duces the con­cen­tra­tion of pre­cious pho­tons. That makes it very hard to snap a crisp im­age of an ex­o­planet.

Un­til a cou­ple of decades ago the only way to elim­i­nate the twin­kle was to place an ob­ser­va­tory above the at­mos­phere, as with the Ke­pler and Hub­ble space telescopes. Now a tech­nique known as adap­tive op­tics is able to sense the in­com­ing light to quan­tify the in­ter­fer­ence caused by at­mo­spheric tur­bu­lence. This in­for­ma­tion is fed back un­der com­puter con­trol to thin re­flect­ing mem­branes that can flex thou­sands of times per sec­ond. This coun­ter­acts the dis­tor­tion by shift­ing the wob­bling light back to its cen­tre, so can­celling the twin­kle. The cor­rec­tive process, akin to that used in noise-can­celling head­phones, has taken decades to per­fect. With it, Earth-based ELTS will be able to re­veal de­tail 10 times finer than the Hub­ble.

There has been no end of spec­u­la­tion about the hab­it­abil­ity of exoplanets but ELTS will be a game changer. Their abil­ity to im­age exoplanets di­rectly raises the pos­si­bil­ity of us­ing spec­troscopy to an­a­lyse the make-up of their at­mos­pheres.

The light spec­trum re­flected by a planet con­tains the sig­na­tures of any gas through which that light has passed. Like a plan­e­tary bar code, this en­ables iden­ti­fi­ca­tion of the el­e­ments and mol­e­cules present in an ex­o­planet at­mos­phere. Some of these el­e­ments and mol­e­cules could re­veal the prospect of life.

One of the most telling is oxy­gen, be­cause it ac­cu­mu­lates in de­tectable quan­ti­ties only through

Adap­tive op­tics will en­able the Earth- based ELTS to re­veal de­tail 10 times finer than the Hub­ble.

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