For years astronomers have been searching for exoplanets — planets orbiting not around our sun but other stars. They have found thousands of them. But the announcement last month of a roughly Earth-sized planet orbiting Proxima Centauri, the nearest star to our solar system, was a surprise.
Proxima Centauri is 4.22 light-years from Earth — a light year is about 6 trillion miles — and is a red dwarf star with only 12 percent of our sun’s mass. It is just 0.1 percent as bright and is a much cooler star than our sun.
The planet is known as Proxima Centauri b or Proxima b, and it appears to orbit in the habitable zone — the range of distance from its sun where water could exist in liquid form. Because the star is a cooler red dwarf, the habitable zone is 25 times closer than the one surrounding our sun. There are very few things that we know for sure about the planet — only its distance from Proxima Centauri, its orbital period, and its minimum mass.
At 4,349,598 miles, Proxima b is very close to its own star, just one-fifth the distance between our sun and its closest planet, Mercury. Proxima b completes one orbit around its star every 11.2 days. The orbit could be circular or it could be very elliptical; we do not know.
Proxima Centauri b is at least as massive as the Earth. It may be in the range of 1.3 times as massive. It could be a gas giant like Neptune, with a thick gaseous envelope, but only a small number of the possible orbits would make this possible. It is much more likely to be a rocky planet like Earth.
Since Proxima b is in the habitable zone, does it have liquid water on the surface? That all depends on if the planet has an atmosphere and whether that atmosphere could allow for liquid water to exist on the surface. Some other important questions we cannot yet answer are: What is the surface temperature? What is the surface pressure? What is the chemical makeup of the atmosphere?
How much water the planet has may depend on its history. If Proxima b formed in its present orbit, it is likely that any water the planet had when it formed has been heated to vapor and lost.
But if the planet formed at a great distance from the star and slowly migrated inward to its present position, it could have been ice-rich and could now have a plentiful water supply.
A third possibility is that the planet lost its formation water, but more was delivered at a later time by comets or asteroids. It is possible that this planet has surface water — we just do not know.
Planets that orbit this close to their parent star are often tidally locked. Because of the strong force of gravity pulling on the planet, the rotational period and the orbital period become the same. In other words, their day and year are the same amount of time.
Tidal locking means the planet keeps the same face toward its star. This may not be good for habitability because the star-facing side would be too hot and the dark side would be too cold. However, it is possible that if the planet has an atmosphere, the heat could be distributed more evenly by winds, making the planet potentially habitable.
Another problem is that red dwarf stars produce powerful flares, which can destroy a planet’s atmosphere and expose its surface to harmful radiation. A magnetic field would give some protection, but we do not know if Proxima b has one.
In the past it was believed that slowly rotating planets could not support strong magnetic fields, but recent research indicates that magnetic fields may actually be supported by convection: hot material near the center of the core rises, cools, and then returns. If this is true, then tidally locked planets may be able to maintain a strong magnetic field for billions of years. Proxima b may therefore have a strong protective magnetic field and yet be tidally locked.
So, is Proxima Centauri b a habitable world? We do not have enough information to answer that question. But it is really nice to have found a planet next door that we can study.