Astronomers studying exoplanets—planets outside of our solar system—have been teasing us with some tantalizing data over the past year. This time 12 months ago, scientists using the Keck Observatory confirmed the discovery of the first Earth-sized exoplanet orbiting within a distant star’s habitable, or “goldilocks” zone, which is the region around a host star within which liquid water could pool on the surface of the planet. Then, nine months later, researchers announced that they found eight new exoplanets all orbiting within this region of their host star, two of which were very similar to Earth.
So these planets are good candidates for hosting alien life, but how do we detect it? Scientists can use indirect methods, such as looking for evidence of biosignatures—substances that could indicate the presence of past or present life—in a planet’s atmosphere. But now, a group of researchers may have come up with a more direct way to hunt for life on exoplanets, having created a colorful archive of “chemical fingerprints,” or reflection signatures, of a range of microbes that inhabit Earth.
“This database gives us the first glimpse at what diverse worlds out there could look like,” study author Lisa Kaltenegger said in a statement. “We looked at a broad set of life forms, including some from the most extreme parts of Earth.”
When scientists observe and characterize distant planets, they examine the starlight that bounces off their surface or chemicals in their atmosphere. Different molecules reflect light in different ways, so by recording the spectrum of wavelengths reflected by something, scientists can detect which substances are present.
So just like how scientists look for the reflection signatures of different gases or organic molecules, for example, in theory astronomers could also look at the reflectance spectra resulting from an organism’s pigmentation, or coloring, if they are abundant enough at the surface. Pigments are chemicals that absorb specific wavelengths of light and reflect all others, and the color we perceive is the result of the combination of reflected light.
But before we can start looking for signs of life based on the imprint left by their pigmentation, scientists needed to first start cataloging the reflection signatures of life on our own planet, which could give us an indication of what diverse life forms could potentially look like. To do this, an international team of researchers, led by scientists at the Max Planck Institute for Astronomy, gathered a collection of 137 different microbes, spanning a range of colors and residing in very different environments, including some of the most harsh and extreme habitats on Earth.
The team then recorded the signatures of these organisms by shining light on them and measuring the reflectance spectra in both optical and near-infrared wavelengths. The results were then compiled into an online biosignatures database that is freely available, and they hope to continue adding to it over time.
While this catalog gives us a fantastic snapshot of the diversity of life that could potentially be detected, unfortunately it will be some time before it can be put to use. That’s because even the most sophisticated telescopes cannot directly measure light from an Earth-sized planet since it is obscured by the bright light of its host star.