Exoplanet Guide

Scienceline’s Guide to the Exoplanets: Anti-Earth

Kepler-438 b: What Earth might have been

May 18, 2020
A fake planet with "Scienceline's Guide to Exoplanets" labeled across its equator. Other planets zoom past behind it.
Your favorite planets, and you didn't even know they existed. [Credit: Curtis Segarra | CC BY-NC-ND 2.0]

Anti-Earth
Kepler-438 b
Discovered by: Kepler
Discovered: 2015
Distance from Earth: ~640 light-years
Size: ~1.1 Earth radii
Surface climate: Neither cold nor hot
Habitability for humans: …perhaps not.

Also known by its even more memorable designation KOI-3284.01, Kepler-438 b is a rocky planet about the size of Earth. Unlike many of our exoplanets, it’s not molten into a death ball by its star’s heat. Indeed, Kepler-438 b is cool enough that, on its surface, liquid water would neither boil away nor freeze to death.

Kepler-438 b is also a planet that shows just how quickly and dramatically our knowledge can turn around, especially in the fluctuating, fast-flowing field of exoplanets.

Kepler-438 b orbits a red dwarf about half the size of the Sun — and Kepler-438 b orbits at about 15 million miles out, less than half the distance between the Sun to Mercury, but a distance that could put it well within the inner edge of that red dwarf’s habitable zone.

Indeed, when Kepler team members announced Kepler-438 b’s discovery as part of a new wave of findings in 2015, scientists — and the public — speculated it might actually have liquid water.

The media called it the “most Earth-like planet ever discovered” and the “most Earth-like exoplanet yet.” Although scientists still aren’t sure how massive Kepler-438 b is, they did know even then that it was roughly Earth’s size, giving them reasonable confidence that you could walk upon its solid, rocky surface.

Astronomers believe that Kepler-438 b receives about 40% more light than Earth. In comparison, Venus gets about 70% more light. If Kepler-438 b had an atmosphere, its average temperature would be around 60 degrees Celsius, or 140 degrees Fahrenheit. That’s warmer than the hottest day ever recorded on Earth, but it’s certainly a temperature humans wouldn’t immediately die in.

More compellingly, it’s easily cool enough for an abundance of water — and perhaps even other things — to exist.

The problem, then, is the radiation.

Mere months after Kepler-438 b’s discovery, another group of scientists from Warwick University in the United Kingdom, looking back upon recent Kepler discoveries, found that the planet’s star spewed out frequent solar flares — ten times the strength of anything the Sun puts out.

What that means is that Kepler-438 b, every hundred days or so, receives a bombardment of charged particles like those in solar wind, combined with a potent potpourri of X-rays and gamma rays. Such radiation would quickly strip away any significant atmosphere Kepler-438 b might have, leaving it barren and exposed to the vacuum of space. Furthermore, without some shielding we haven’t identified, it would utterly sterilize any life lucky enough to evolve on its surface.

Forget “Earth-like.” It’s likely that, far from a paradise of alien life, Kepler-438 b resembles a cooler version of Mercury.

In other words, Kepler-438 b can tell us a lot about how many things come together for Earth to be the “blue marble” that it is. If it weren’t for Earth’s atmosphere or Earth’s magnetic field to deflect many of our own Sun’s charged particles, Earth would be far less hospitable.

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