A report from the European Southern Observatory highlights some of the clever techniques that scientists use to gather evidence that seems, at first glance, unobtainable. In this case, astronomers have been able to obtain some information about the atmosphere of an exoplanet, orbiting a star about 50 light-years away from Earth in the constellation Boötes.
Light does more than just illuminate things. Most sources of light are actually mixtures of light of various wavelengths, and scientists are able to use instruments to split that mixture into a spectrum showing its various components. Different sources of light give of different, characteristic spectra. The compositions of distant stars can be determined by the spectra of their light. Elements and compounds in stars, nebulae, and so forth, can either emit or absorb light of characteristic wavelengths. Teasing out those fingerprints from light gives us information about the nature of the light source.
This approach seems straightforward enough when applied to stars, but what about planets orbiting distant stars? Ordinarily, the light from the planet itself is too feeble, when viewed from Earth, to distinguish it from the parent star. An analogy would be trying to see a candle flame next to a spotlight, from a distance. The star’s light is just too overwhelming. However, the ESO team of astronomers have developed a technique that allows them to separate the planet’s light from the star’s.
The light reaching us from distant star systems is composed of both the light from the star and the light of the planet (or planets). It is difficult, when looking at a single spectrum, to determine which features of the spectrum belong to the planet, and which to the star. However, the astronomers kept in mind that the planet moves. This means that for half of its orbit, it is accelerating towards the Earth, and accelerating away for the other half. When a light source is accelerating relative to Earth, the wavelength of its light shifts, albeit slightly. By observing the light from the star system for a long enough period of time, the astronomers were able to identify those components of the combined star + planet light that were shifting in the regular way associated with an orbiting planet. By cutting out just those parts of the light, they were able to isolate the planet’s light from the star. Now, that spectrum can be examined for clues about the planet’s nature.