A potentially habitable, Earth-like world has been discovered around the nearest star from the Sun, Proxima Centauri. It sounds pretty cool when you say it like that. While it is a great discovery, there are reasons to be cautious.
To begin with, this particular planet, known as Proxima B, was discovered using the Dopper method rather than the transit method. This means it was discovered by looking at how the light from Proxima Centauri shifts due to the motion of its surface. While such Doppler effects can be due to the gravitational influence of an orbiting planet, it can also be caused by things like solar flares. Sometimes this leads to a false positive, so we really need more data to be sure it’s really a planet. Assuming the data holds up, calling the planet “Earth-like” is a bit optimistic. Since the Doppler method only measured stellar motion towards us and away from us (radial motion) it only gives the minimum mass of a planet. If the planet’s orbit is highly tilted relative to us, then its mass will be larger. The quoted size of 1.3 Earth masses is actually the low end, and it’s more likely to have a mass of 2 – 3 Earths, making it more of a super-Earth planet.
The planet is also in Proxima Centauri’s habitable zone, which again means less than you might think. The “habitable zone” of a star is just a simple calculation of an distance where liquid water could exist on a planetary surface. Actual surface temperature depends not only on distance, but also atmospheric density and composition. One need only look at the variation between Venus, Earth, and Mars to see how much that matters. Since Proxima Centauri is a red dwarf, there are other challenges to habitability. For one, any planet in a red dwarf’s habitable zone is likely to be tidally locked, with one side under constant noon while the other side is in constant night. Such planets are likely to experience constant extremes of temperature, rather than a reasonable Earth-like variation. Then there is the fact that red dwarfs like Proxima Centauri are quite active, with large solar flares and bursts of x-rays that could fry a close planet like Proxima B.
All that said, there is some reason to be optimistic. Computer simulations of the planet’s orbit indicate that it likely formed farther away from Proxima Centauri, and therefore is likely to have plenty of water. It’s mass is large enough that it could have a strong magnetic field like Earth due to an iron core and geologic activity, and this could protect the planet’s surface from solar flares and x-rays. Calling the planet potentially habitable is not too much of a stretch. There is a chance that if its mass is on the low end and conditions are favorable it could look somewhat like Earth.
But the real draw for Proxima B is that it is only 4 light years away. That’s still about 180,000 times the average distance from Earth to Mars, but it is close enough that we can imagine sending a space probe to Proxima B. The proposed Project Longshot mission envisioned just such a mission that would take about 100 years. It’s a long mission, but humans have undertaken century-long projects before. Just as the call of the Moon led to the Apollo mission, and Mars inspires near-future missions, Proxima B inspires a mission to the nearest star.
That’s the power of a planet like Proxima B. It allows us to hear the call of the wild.