Exoplanets: Why Earth remains unique

There are many reasons to be excited about the recent discovery of water vapour in a distant exoplanet’s atmosphere. This “super-Earth", K2-18b, eight times the mass of Earth, is now the only planet orbiting a star outside the solar system known to have both water and temperatures that could support life, according to an official statement from University College London (UCL). A “super-Earth" is a planet with a mass between that of Earth and Neptune.

Astronomers from UCL’s Centre for Space Exochemistry Data used archival data captured from the Hubble Space Telescope in 2016-17 and developed special algorithms to analyse the starlight filtering through K2-18b’s atmosphere. Apart from the molecular signature of water vapour, the findings also indicated the presence of hydrogen and helium in K2-18b’s atmosphere.

When a planet moves around its star, some of the light from the latter gets blocked, but some of it gets filtered if the planet has an atmosphere. According to a Forbes report, the starlight that filters through the atmosphere creates “absorption or emission lines" that can be detected by sophisticated observatories. K2-18b orbits the cool dwarf star K2-18, which is approximately 110 light years away from Earth.

But scientists are sceptical about describing K2-18b as a “habitable" planet. Part of the reason is the high activity of its dwarf star, which means the planet is likely to be exposed to more radiation, according to the UCL statement. The viability of K2-18b and other “potentially habitable" planets can only be ascertained by future space missions and instruments like the James Webb Space Telescope, which is due to be launched in 2021 and will be able to characterize an exoplanet’s atmosphere in greater detail. Similarly, the ARIEL (Atmospheric Remote-sensing Infrared Exoplanet Large-survey) space mission, expected to be launched by the European Space Agency in 2028, will observe 1,000 exoplanets in detail.

According to its official website, ARIEL will be the first mission to measure the chemical composition and thermal structures of exoplanets. For some select planets, ARIEL will also perform an in-depth survey of cloud systems and study seasonal and daily atmospheric variations. During its initial four-year mission, ARIEL will study what exoplanets are made of, how they were formed and how they evolve.

Other possible missions in the future to detect exoplanets could use the starshade technology. According to the US’ Nasa Jet Propulsion Laboratory website, this would involve the use of two spacecraft. One would be a space telescope and the other would fly some 40,000km in front of it, carrying a large starshade that would block the light from a star, “allowing the telescope to get a clearer glimpse of any orbiting planets".

As Pat Brennan, a veteran science writer for Nasa’s Exoplanet Exploration Program, wrote in an article (“Is Earth An Oddball?") in August, “Future space telescopes could examine the atmospheres of distant, rocky worlds for signs of oxygen, methane or carbon dioxide—in other words, an atmosphere that reminds us of home." In a nutshell, finding Earth-like planets or a unique planetary system like ours is not going to be an easy task.

Leaving Earth will be the inevitable result of our quest to find life beyond the boundaries of the solar system. But we are still light years away from that outcome.