It is often discussed within the community of exoplanet scientists that a danger lies in the description of intriguing exoplanets as “Earth-like.”
Nothing discovered so far warrants the designation, which is pretty nebulous anyway. Size and the planet’s distance from a host star are usually what earn it the title “Earth-like,” with its inescapable expectation of inherent habitability. But residing in a habitable zone is just the beginning; factors ranging from the make-up of the planet’s host star to the presence and content of an atmosphere and whether it has a magnetic field can be equally important.
The recent announcement of the detection of a planet orbiting Proxima Centauri, the closest star to our own, set off another round of excitement about an “Earth-like” planet. It was generally not scientists who used that phrase — or if they did, it was in the context of certain “Earth-like” conditions. But the term nonetheless became a kind of shorthand for signalling a major discovery that just might some day even yield a finding of extraterrestrial life.
Consider, however, what is actually known about Proxima b:
- The planet, which has a minimum mass of 1.3 Earths and a maximum of many Earths, orbits a red dwarf star. These are the most common class of star in the galaxy, and they put out considerably less luminosity than a star like our sun — about one-tenth of one percent of the power.
- These less powerful red dwarf stars often have planets orbiting much closer to them than what’s found in solar systems like our own. Proxima b, for instance, circles the star in 11.3 days.
- A consequence of this proximity is that the planet is most likely tidally locked by the gravitational forces of the star — meaning that the planet does not rotate like Earth does but rather has a daytime and nighttime side like our moon.