editorial The weight of a world What’s in a name? Would a super-Earth by any other name smell as sweet? It should, but maybe it wouldn’t get as many clicks.

t’s an inconvenient truth that the most Neptune. This advance is huge, as we really (as we know it), Carl Sagan et al. explained common type of planet in the Galaxy want to target terrestrial planets. As K2-18 in an Article that the detection of oxygen I(Universe?) does not exist in our Solar b is in the habitable zone, it means we can and methane in disequilibrium would be System. As they have a mass between those get down to business and probe terrestrial necessary. Michael Mendillo, Paul Withers of Earth and Neptune, they are collectively atmospheres in this mass region. and Paul Dalba more recently argued in known as ‘super-Earths’ or ‘sub-Neptunes’. From a perspective of habitability, we a Perspective that a detection of O+ in an Their Latin prefixes (super = over; sub = specifically want to focus on exoplanets with exoplanet’s ionosphere would be sufficient as below) were meant to define the planets as radii less than 1.5 times that of the Earth. As a sign of oxygenic photosynthesis. To search being bigger than Earth and smaller than explained — in an entertaining fashion — in for biomarkers on rocky exoplanets, we will Neptune. Over time, however, the terms a blog by Elizabeth Tasker, planets larger need the James Webb Space Telescope and developed an additional meaning connected than that tend to be less dense than Earth the Atmospheric Remote-sensing Infrared to the planet with which they are compared, and in possession of a warmer cloak of light Exoplanet Large-survey (ARIEL) mission. so super-Earth is considered a synonym of gases. Indeed, both papers additionally When these observatories come big rocky planet and sub-Neptune of small report the presence of hydrogen on K2-18 online, it will be even more important icy giant. In reality, for many of them we do b. According to Tasker, the hydrogen- to use precise scientific terminology. not know their exact nature, whether they dominated atmosphere invalidates any talk At a recent Exoclimes V conference in are rocky, icy or even ocean worlds. Let of habitability, as the surface temperature Oxford, UK, Joseph Harrington suggested alone if they have an atmosphere or not. is likely to be way too high. Let’s be clear: a kind of Torino scale for the reporting of But the term Earth is loaded with humans could not survive on K2-18 b. biosignatures. The Torino Impact Hazard meaning. And should an article also include Moreover, we do not know if K2-18 b has a Scale for the public communication of the phrase habitable zone, the public surface for a human to stand on, or indeed a potential asteroid impact ranges from immediately thinks that we are talking lie on, given the pressure is so high. Still, zero to ten and is colour coded. Thus, a about Earth 2.0: a rocky planet with surface not many news stories focused on the score of zero lies in the white zone, with water and an atmosphere and perhaps a inhospitable nature of a planet in close orbit a negligible threat of collision or minimal more striking sunset than ours. And who around an M dwarf with high X-ray and impact on life, and eight to ten lie in the can blame them? However, the habitable ultraviolet activity. XKCD, however, totally red zone, indicating certain collision, zone is most commonly defined as the nailed it in their comic. with ten meaning that we can expect a region around a star in which the surface From a scientific point of view, however, hit with global-scale consequences, such of an Earth-like planet would be temperate exoplanets such as K2-18 b are a mine of as the Chicxulub impact that wiped out enough to support liquid water. Note that information. They are common in size, the dinosaurs. A level-three asteroid has the Moon, Mars and Venus are also in the except within the Solar System. NASA’s a 1% chance of collision, and that’s when habitable zone of our Sun. To maintain Kepler mission detected hundreds of astronomers are brought in to monitor the liquid water on the surface, the Earth exoplanets in this mass range, and hundreds object. A similar scale for the exoplanet needs greenhouse gases in its atmosphere more are expected from the Transiting community was enthusiastically received, so to warm it up. Otherwise the equilibrium Exoplanet Survey Satellite (TESS) mission. we can hope for some guidance soon. temperature of the Earth would be 257 We do not know whether K2-18 b is a large Calling for more care in the language K — well below the freezing point of water terrestrial planet or a small ice giant. Thus, of reporting takes nothing away from (under standard atmospheric conditions). it could equally be called a sub-Neptune these amazing discoveries. There is simply When two papers, a preprint by Björn rather than a super-Earth. We should drop no need to be ambiguous (at best) or Benneke et al. and a Letter by Angelos these names altogether. In their Comment, deliberately misleading (at worst). For over Tsiaras et al., reporting the detection of William Moore et al. outline the need for two years, this journal has minimized the water vapour in the atmosphere of exoplanet precision: “terrestrial planet in a temperate use of unqualified instances of super-Earth, K2-18 b — eight times the mass of the Earth orbit” is not the same as “Earth-like planet sub-Neptune, hot-Jupiter and so forth, and therefore technically a super-Earth — in the habitable zone”. It’s a view supported though we could do better. Similarly, appeared online within a day of each other, in a Comment by Tasker et al., yet more than the habitable zone has too many it was no wonder that over 3,800 news two years have passed since these articles implications and needs to go. Let us all outlets covered the results. Finding water were published and the situation has be more vigilant. ❐ in the atmosphere of a temperate exoplanet not improved. is hugely exciting. It is also technically And let’s not forget that many molecules challenging, and heralds our ability to study could be abiotic as well as biological in Published online: 7 October 2019 the atmospheres of exoplanets smaller than origin. To even suggest the presence of life https://doi.org/10.1038/s41550-019-0923-8

Nature Astronomy | VOL 3 | OCTOBER 2019 | 875 | www.nature.com/natureastronomy 875