An international team of researchers claims that the link between the colour of a planet and its surface features can be used to prioritize which newly found exoplanets, especially rocky planets with clear atmospheres, should be studied in-depth for signs of life. The work provides an important link between Earth-based geomicrobiology and observational astronomy.

A huge number of exoplanets have been discovered in recent times – just over 800 confirmed examples are known today, with more than 2000 candidates waiting to be confirmed. Of the candidate exoplanets, it is difficult to decide which ones are the most likely to harbour life.

Home sweet home?

"What is now observed is that smaller Neptune-sized planets are, in fact, far more abundant than larger Jupiter-sized ones. This is exciting and one feels that it is only a matter of time before the same can be said for Earth-sized planets around other stars. The question then naturally arises as to how one could characterize these rocky planets to check for their potentially habitability," explains Siddharth Hegde of the Max Planck Institute for Astronomy in Germany. He and colleague Lisa Kaltenegger from the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, in the US have explored how filter photometry can be used to pinpoint Earth-like exoplanets and study their atmospheric bio-signatures – whether they have aerobic or anaerobic atmospheres. Looking at the diversity of life on Earth, even under extreme conditions, the researchers wonder whether planets around other stars with extreme surroundings could also harbour some form of life.

In astronomy, photometry is a way of measuring the flux of the electromagnetic radiation of an astronomical object. "Filter photometry basically means that you split the collected light [from a celestial object] only into a few wavelength bins that are defined here by the commonly used filters in the visible called 'B, V, I Johnson–Cousins filters' [or blue, green and red colour bins]," explains Hegde. The advantage of this approach is that lots of photons are gathered per bin, meaning a good signal-to-noise ratio is achieved – which, in turn, means that it may be possible to characterize dimmer planets. The researchers use this method to identify planets that have surfaces similar to those on Earth that harbour life. This is done by plotting the blue–green versus blue–red bins using customized filters, creating what is known as a "colour–colour diagram". While the technique does not provide the finer details of a planet, it can very easily be used to put together a follow-up prioritized "target list" of planets that should be studied in detail with spectroscopy.

True colours

A way of looking for these extreme environments is to study the "albedo" of a planet – its reflectivity as a function of wavelength. For example, snow has a high albedo, meaning that it reflects well, while water has a low albedo and so does not reflect as well. A previous study, conducted in 2003, compared the colour–colour diagrams of rocky and Jupiter-like planets in our solar system to see whether they were the same – they were not. That study concluded that a colour–colour diagram can be used to make a first-order basic characterization of a planet's nature. Hegde and Kaltenegger extended this idea to rocky exoplanets based on the assumption that these habitats best determine the environmental limits for harbouring Earth-type extremophiles.

Going to extremes

An extremophile is an organism that exists in physically or geochemically extreme conditions – such as extreme temperature, radiation, pressure, dryness, salinity or pH – that are detrimental to most other life-forms on Earth. "By splitting the light from a hypothetical planet, with a surface covered with a material that can harbour extremophiles on Earth, into the three filter bins, we found that those planets fall into a tight band when plotting a colour–colour diagram," says Hegde.

The method is similar to another already used by exoplanet hunters who look for the "red-edge" – a telltale sign of vegetation – in the spectra of planets. This is a large and abrupt change in the absorption of light by plants that occurs at about 700 nm. At shorter wavelengths, chlorophyll absorbs very strongly and therefore plants reflect little light; above 700 nm, chlorophyll does not absorb light, which means that leaves are able to reflect much more sunlight back into space. Combining such spectral readings with colour–colour diagrams could clearly indicate if a planet has any Earth-like life, or is capable of harbouring it.

In the future, the researchers are keen to study possible changes in a planet's atmosphere caused by different kinds of extremophiles that might inhabit its surface – for the moment, their model assumes the extremophiles do not affect the atmosphere significantly. "Maybe, with the help of biologists who culture such extremophiles in the lab, we can find out if there are gases in the atmosphere that can tell us whether such surfaces really harbour life," muses Hegde.

A paper on the work is available on the arXiv preprint server.