Although hundreds of exoplanets have been discovered astronomers haven't yet found one that is
Twenty years ago, astronomers didn’t know for certain whether there were planets in orbit around any stars other than the Sun. All that changed in 1995 with the discovery of a planet orbiting the star 51 Pegasi. Today over 800 exoplanets are known with thousands more candidates having been identified (see www.exoplanet.eu for a complete catalogue).

The reasons for the huge advance are twofold: first, technology has improved to the extent that the tiny influences that a planet has on its parent star may now be measured; and second, advances in computing mean that vast quantities of data may now be automatically scanned to pick out the rare signals that indicate the presence of an exoplanet.

Many people assume that astronomers ‘see’ exoplanets directly through their telescopes, perhaps as a pale blue dot sitting alongside a distant star, possibly even observing shifting cloud patterns on its surface. The reality is even more miraculous: in the vast majority of cases, all that is seen is a single, stationary pin-point of light.

Yet by measuring the brightness of that speck of light, astronomers may see a tiny repeated dimming, as though the star is winking at us, indicating the passage of an exoplanet in front of the star. By measuring Doppler shifts in the star’s spectrum, they may infer the star is wobbling back and forth, as it is tugged by the gravity of an exoplanet.

From the depth of the ‘wink’ and the amplitude of the ‘wobble’, the size and mass of the unseen exoplanet can be determined. Knowing this, its density and surface gravity can be calculated, so giving an indication of its bulk composition.

It’s also easy to determine the length of the exoplanet’s year, from the interval between repeated ‘winks’ and ‘wobbles’. From this, the distance of the exoplanet from the star and its mean temperature may be calculated, and therefore whether or not liquid water can exist on its surface may be determined. All this is from seeing a pin-point of light.

But the extrapolation doesn’t stop there. By carefully plotting the tiny variations in light as the exoplanet orbits the star, and passes behind it, it’s possible in some cases to determine the differences in temperature between the night-side and day-side of the planet and to infer seasonal variations in its cloud cover.

Most remarkably of all, when the exoplanet passes in front of the star, a fraction of the star light reaches us after passing through the exoplanet’s atmosphere. By studying the differences between the in-transit and out-of-transit spectra, it’s possible to infer the composition of the exoplanet’s atmosphere.

Ultimately, it is measurements of this kind that will tell us whether or not humankind is alone in the Universe. Many astronomers are confident that the detection of biomarkers in the spectrum of the atmosphere of an Earth-like exoplanet will happen within the next 20 years, if such life actually exists elsewhere in the Galaxy. It may only be a green slime that could be scraped off a rock with a finger nail, but it would completely transform humanity’s view of our place in the Universe. And all that is possible merely by observing a pin-point of light through a telescope.

For information about the most successful ground-based transiting exoplanet search, see this article about SuperWASP – the Wide Angle Search for Planets, and for information on the NASA mission which is looking for transiting exoplanets from orbit, see this article about the Kepler satellite:

Almost 300 exoplanets have now been confirmed by the transit method. Most are hot Jupiters, but increasingly a number of Neptune-sized, or super-Earth sized planets, have now been found too. Some of the planets discovered are in the star’s habitable zone – the region of space around the star where the temperature is such that water might exist in liquid form on the surface of a planet that lies there.

Clearly such planets are of great interest as offering the potential for harbouring life. Discovering whether transiting exoplanets in a star’s habitable zone might actually host alien biospheres is (amazingly) potentially within reach of observations.


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