Excited reports of ‘habitable planets’ need to come back down to Earth

In 1950, physicist Enrico Fermi famously asked, “Where are they?” as a kind of lament about the lack of observational evidence for alien intelligence in our universe. Today, the question is still asked in the context of the always-hoped-for discovery of other worlds like our own, with the thought that maybe, just maybe, we will finally find those aliens.

Against this backdrop, advances that have occurred in the past 20 years in the field of exoplanet discovery have excited the imaginations of scientists and enthusiasts alike.

When, the question goes, will we finally discover a planet that can sustain life? When will we discover Earth 2.0?

The impatience associated with this question has led many in the media and even some in the scientific community to make premature declarations that an “Earth analogue” has been discovered. But when exoplanets are discovered, claims that they are similar to Earth are based on, at best, optimistic modelling and, at worst, sensationalism.

Many such claims have been made on the basis of invented ranking systems that use the observed properties of exoplanets to extrapolate how Earth-like a planet might be. Unfortunately, these systems must make extremely simple – and almost certainly incorrect – assumptions about the characteristics of the planets they are trying to describe.

Ranking habitability: not so easy
Before any exoplanets had been discovered at all, certain astrophysicists proposed that each star had an associated zone around it that came to be known as the “habitable zone”. This zone is at a distance from every star where a hypothetical Earth twin would have an average surface temperature between the freezing and boiling point of water. Too close and you exceed 100°C; too far and you drop below 0°C.

But if a planet has an atmospheric composition different from Earth’s, its true surface temperature is likely to be completely different.

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