It isn’t easy to spot planets far away in our galaxy – normally we can only infer their presence, from the effect they have on their host stars. But now a group of astronomers has demonstrated an easier way to study distant worlds, by detecting the first visible light reflected off an exoplanet.
According to group member and PhD student Jorge Martins at the University of Porto in Portugal, seeing the exoplanet like this was rather like looking at a streetlamp one hundred metres away, and being able to glimpse a moth flying right next to it.
‘This detection is equivalent to detecting the light from the lamp, reflected on the wings of the moth,’ he said.
Since the early 1990s, astronomers have spotted nearly 2 000 planets outside our solar system. Mostly these exoplanets have revealed themselves by the changes they inflict on their host stars – perhaps a faint shadow, created as they pass in front of the star; or perhaps a slight wobble, caused by their gravity pulling at the star as they orbit around it.
Getting any detailed information about an exoplanet is more difficult. Sometimes when an exoplanet passes in front of a star it is possible to identify colour changes that suggest the presence of certain chemicals. So far, exoplanets have been found that contain carbon dioxide, methane and even water – the last one being particularly interesting, since it is thought to be one of the precursors of life.
But in order to study an exoplanet, astronomers do not always want to wait for that rare occasion when it passes in front of its host star. After all, if they want to study a planet in our own solar system, they simply look at it directly though a telescope, and examine the light reflecting off it from our sun.
‘This detection is equivalent to detecting the light from the lamp, reflected on the wings of the moth.’
Jorge Martins, University of Porto in Portugal
In fact, that is pretty much how Martins and his colleagues have now studied 51 Pegasi b, a Jupiter-sized exoplanet orbiting the star 51 Pegasi, which is 50 light years away in the constellation of Pegasus.
‘Detecting planets in reflected light is a fundamental goal of exoplanet research,’ said Dr Quinn Konopacky, a leading astronomer at the University of Toronto in Canada who was not involved in the study. ‘This detection of reflected light from a Jupiter-like planet is a critical first step in reaching that goal.’
Martins and colleagues’ demonstration was not as easy as pointing a telescope in the right direction. 51 Pegasi b is about four million times farther away than our closest planetary neighbour, Venus. Worse, it reflects only about one ten-thousandth of the light emitted by its host star.
Cross correlation function
To pick out the faint signal of 51 Pegasi b, then, the researchers turned to a mathematical function known as a cross correlation function, which could seek out the spectral colour lines indicative of a planet and then amplify them. They applied the function to spectral data taken by the European Southern Observatory’s La Silla Observatory in Chile.
The technique allowed them to view the reflected light spectrum of 51 Pegasi b, and extract a few parameters from it. These included estimates of the planet’s mass and its reflectivity, or ‘albedo’.
Dr Nicolas Cowan, a leading astronomer at Amherst College in Massachusetts, US, who was not involved with the study, said that these estimates do not totally agree with those calculated by independent means, but calls the demonstration ‘exciting’ nonetheless. ‘Not so much because of the specific detection for this planet … but rather because the technique is promising and this is the first time that people have used it to look for reflected light,’ he added.
The demonstration of the technique is not the first result to come from the group as part of the EXOEarths project, which is backed by the EU’s European Research Council. In 2012, for instance, the group announced the discovery of a planet orbiting in the Alpha Centauri star system that was smaller than any discovered previously.
The latest technique is expected to reap more substantial rewards when it is used with bigger telescopes, such as the ESO’s Very Large Telescope or its forthcoming European Extremely Large Telescope, both in Chile.
‘This method will eventually be used to measure reflected light from many other exoplanets,’ said Dr Cowan. ‘Exoplanetology is still in its infancy, and these sorts of sketchy first detections quickly give way to robust results.’
Dr Konopacky said the researchers are ‘pushing current instruments to the limit’.
‘In the future, we will have new instruments that are specifically designed to detect reflected light from planets more like earth,’ she added.
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