How Extremely Large Telescopes will reveal 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.
Andrew Grey discovered a four-planet solar system 600 light years from Earth. He’s not a professional astronomer. He’s a 26-year-old car mechanic from Darwin. His persistence in trawling through a thousand or so light curves – star brightness graphs – has been rewarded big-time. On live television, to boot.
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 minuscule 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 ‘super Neptunes’ (planets like Neptune that are gassy on the outside with a solid core), ‘super Earths’ (giant rocky planets whose mass is greater than our own but less than the likes of Neptune), and ‘Earth-like planets’ (roughly the same mass as our own, orbiting in the ‘goldilocks’ zone where liquid water can exist); about 5% of analysed stars have been found to have such planets, putting the number of possible Earth-like planets in our Galaxy well into the tens of billions. Smaller worlds, below the current level of detectability, must be there, too.
While newly found planets are rolling off the production line, the truth is we have really only scratched the surface of what we can learn. But that is about to change – very radically. Enter the age of the extremely large telescopes.
The bigger the better
The world’s optical astronomers suffer from aperture fever; they crave ever bigger mirrors for their telescopes. This is not mere megalomania, and not even merely the wish to see more distant celestial objects. It’s mostly about how much light is at your disposal, and what clever stuff you can do with it.
One of the really clever things that can be done with larger telescopes is to see exoplanets directly, rather than relying on how they nudge or shade their star.
In the 1970s and 1980s, the astronomical world saw a proliferation of telescopes in the 4-metre class. The 1990s saw the introduction of 8-10 metre giants. This new generation of so-called extremely large telescopes, or ELTs, now being built have mirrors more than 20 metres in diameter. Lenses crafted from single pieces of glass would be impossible in these sizes; but by various techniques of segmenting, and aligning individual pieces of glass with computer-controlled fingers to replicate a single reflecting surface, the size problem can be solved.
The ELTs will chart new territory. They will peer back into the early universe to reveal its history and shed new light on mysteries like the origin of black holes, dark matter and dark energy. Just as the 16th century explorer Ferdinand Magellan – after whom one of the telescopes is named – had no idea of what he was about to discover as his ships sailed into the Pacific, we don’t know what lies ahead. But among the exciting things that will come into view are the exoplanets.
An individual ELT, of course, also comes with an ELPT – an extremely large price tag, typically in the region of a billion dollars. Funding at this level demands large international collaborations. Three groups are actively involved in building ELTs. Two are US-led: the Giant Magellan Telescope (GMT), with which Australia is partnered, and the Thirty Metre Telescope (TMT). The third is the European Extremely Large Telescope (E-ELT).
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.
Adaptive optics will enable the Earth-based ELTs to reveal detail 10 times finer than the Hubble.
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.
Looking for life
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 biological processes – most notably photosynthesis. Moreover, because it reacts so readily with other molecules, oxygen has to be continuously replenished to remain in circulation. Our own planet clearly signals the presence of life by the fact oxygen accounts for almost 21% of the atmosphere.
The presence of oxygen in a planet’s atmosphere, however, is by no means evidence of complex life forms; it can be produced by single-celled organisms, like the cyanobacteria thought responsible for the initial oxygenation of Earth’s atmosphere some 2.3 billion years ago. Biomarkers for multi-celled organisms are more subtle. Whether such signatures might be detectable at interstellar distances is a hot topic in astrobiology. Some possibilities do exist: for example, the chlorophyll content. Vegetation produces a characteristic spectral profile. This so-called ‘vegetation red edge’ is already used to map our own planet’s resources from space.
How might we react to the unequivocal detection of rudimentary life beyond our planet? Whether life exists elsewhere in space is one of the biggest questions of our time. Even the discovery of single-celled organisms would have far-reaching implications. But the finding that really would be overwhelming is unequivocal evidence of an intelligent civilisation. The sociocultural impacts of such a discovery would be profound. Science, technology, ethics, politics and religion – all will undergo major shifts as we come to terms with a completely new perspective: we are not alone.
The way ELTs might reveal that knowledge is by finding so-called technomarkers. There are chemicals that can only be introduced into a planet’s atmosphere in significant amounts by industrial processes. They include well-known offenders such as chlorofluorocarbons. Eventually ELTs should allow us to detect these tell-tale pollutants in the atmospheres of distant planets. The irony is inescapable: extra-terrestrial intelligence discovered because aliens were trashing their planet, just as we are trashing ours.