TELESCOPES TO SEE EXOPLANETS

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.

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.