The search for life on other planets is a major topic in planetary sciences. For many years we have been looking for that light which hopefully one day will help us to find life outside Earth.
So far, the presence of life on other bodies inside the Solar System has not been proven. Therefore, the search has been extended to planets outside our system to raise the number of potential targets. It has been about two decades since the discovery of the first extrasolar planet and the list continues increasing rapidly with more than 1800 extrasolar planets known up to date. However, from all these planets, Earth still remains as the only planet known to support life. For this reason, our home planet serves as a key reference to find planets with conditions that are suitable for life.
The interest given to the search for life has been focused on planets that are at a distance where moderate surface temperatures and pressures exist at the planet’s surface for water to be found in its liquid state. The region where this may be possible is known as the habitable zone (see Figure below). Also, various molecular species that compose the atmosphere of the planet may give a hint since they are considered to be indicators of life as they are produced by either direct or indirect biological activity.
The nearest extrasolar planet might be found at a distance of 4.24 light years (1) away from the Solar System where the star Proxima Centauri is located. Now, the tricky question is how are we able to detect the characteristics that we are hoping to find of a planet from such great distances? The answer lies on the light we are able to measure through telescopes.
By using different observational techniques, it is possible to constrain some of the physical properties of the planet. However, the most direct way to obtain information on the atmospheric state and composition of a planet is through spectroscopy since the only quantity that can be measured from a distant planet is the radiation of photons, that is light.
The interaction of light particles with the gases or molecules located in the atmosphere can generate absorption and emission lines in a spectrum. These lines are considered to be fingerprints since each one of the molecular species absorb and emit light at unique wavelengths. The fingerprints are spread along a spectrum and they can be compared to Earth’s spectral signatures (see Figure below) to identify the molecular specie present in the atmosphere. The technique used to measure the spectrum of a distant planet is based on transiting planets in which a planet as seen from Earth passes in front of its host star. The light originating from the star travels through the planetary atmosphere and gets absorbed and emitted by molecules producing the fingerprints that we are needing to compare to those of Earth. The light can also be measured directly from the planet as its surface reflects and emits the incoming photons from the star.
Most of the planets detected so far through current observational technology are of similar size to Jupiter and belong to the type known as hot Jupiters (2). Nevertheless, the first extrasolar planet lying in the habitable zone and of a size similar to Earth’s (1.1 Earth-radius) was already confirmed less than one year ago and it still awaiting for characterization. This finding brings us a potentially habitable planet. With the incoming new generation of ground-based telescopes as well as a number of telescopes to be launched to space, a larger number of Earth sized planets are hoping to be detected in the habitable zone that will hopefully increase the probabilities of finding the spectral fingerprints that will match that of Earth’s. By now, we continue our search for that light that will serve as the key to detect life on other planets.
(1) A light year is a unit of lenght used often in astronomy to express the vast distances of the Universe. It is the distance that light can travel in one year, which is approximately 10 trillion kilometers.
(2) They are extrasolar planets very similar to Jupiter in size but they have higher temperatures because they orbit very close to their parent stars.
Mayte Vasquez (@maytevasquezmae) is an astrophysicist working at the German Space Center (DLR). She is also a member of the GalileoMobile project and has traveled to India, Bolivia, Brazil and the Dominican Republic to take astronomy to local schools. She has been in the topic of habitable planets for almost seven years for which she is modeling and analyzing the spectra of exoplanets that are similar to Earth by varying their atmospheric conditions. She is passionate about finding life on other planets and sharing astronomy with teachers and students.