Stop falling in love with the latest Earth-like planet

Exoplanets are easy to love. There are cold ones, super ones, and multi-sunned ones, as the snazzy NASA travel posters below illustrate. The planets that get the most attention are tantalizingly similar to Earth. This week we learned about the latest set of Earth-like worlds , which orbit a tiny star called TRAPPIST-1 that’s only 40 light-years away.

Despite how Earthy the TRAPPIST planets may sound, I implore you to follow a simple rule: Don’t fall in love with individual exoplanets.

This is nothing against the discovery, which was reported 2 May in Nature. It’s just that until a next-generation instrument such as the James Webb Space Telescope (JWST) probes individual planets in detail, astronomers simply don’t know enough about any given exoplanet to confidently call it Earth-like.

Measurement uncertainties are one reason to take the particulars of a newly discovered exoplanet with a grain of salt. Kepler 452b, pegged last year by NASA as Earth’s bigger, older cousin , is somewhere between 1.4 and 1.9 times the size of Earth. Yet where the planet sits within that range is extremely important, as recent research suggests that most worlds with a diameter at least 1.6 times that of Earth’s aren’t rocky . Similarly, any lack of precision in measurements of a planet and its parent star complicates calculations to determine whether the world sits within the liquid water–friendly habitable zone.

Then there are factors that most exoplanet detection methods simply cannot deduce. The new study reveals that the innermost TRAPPIST planet receives about four times as much stellar radiation as Earth does from the Sun; the middle planet receives more than twice as much. But we don’t know the planets’ composition and reflectivity, which dictate how hot each planet gets. In addition, the planets are tidally locked, with one side permanently illuminated and the other in perpetual darkness. Astronomers aren’t sure whether that’s a nonstarter for habitability. Finally, these planets race around a star that, despite having barely enough mass to fuse hydrogen, was apparently much hotter in its younger years.

Again, none of this is an indictment of the new paper. The same reasons for skepticism about the Earthiness of these planets are what make the implications so exciting. Before this discovery many scientists thought planets couldn’t form around such wimpy stars; the fact that they can means there are probably billions of similar planets in the galaxy. This ability to extrapolate is what motivated William Borucki to develop NASA’s Kepler mission. He has often said that the mission was never about individual planets. The goal is using the detection of potentially Earth-like planets to determine how common they are throughout the galaxy.

The TRAPPIST trio have the additional advantage of being located just down the block in cosmic terms, making them good targets for the JWST and other telescopes to probe the planets’ atmospheres. Once we can make such in-depth observations, individual planets become far more interesting.

One day, probably in the not-too-far future, astronomers will identify a planet with a combination of size, mass, and temperature that’s about as Earthy as it can get. The unwieldy name of the planet will trend on Twitter and find its way onto a travel poster. And that’s fine. But consider celebrating the big picture instead: Finding one possibly habitable planet in the incredibly tiny segment of the cosmos we can survey makes it much more likely that truly Earth-like worlds are out there.