By Jonti Horner, University of Southern Queensland and Tanya Hill, Senior Curator (astronomy) - Museum Victoria
After years of searching, an international team of astronomers says they’ve found definitive evidence of a planet orbiting Proxima Centauri, the closest star to the sun. The details are published in Nature today, and this is potentially the single most exciting exoplanet discovery to date.
The planet, named Proxima Centauri b (Proxima b for short), is probably slightly more massive than the Earth, and is about the right distance from Proxima that it could have liquid water on its surface.
Proxima Centauri is a faint red dwarf star, just 4.24 light years from Earth. Despite its proximity, the star is too faint to be seen with the naked eye, which leaves its neighbour, Alpha Centauri, as the closest star that can be seen without a telescope.
Proxima is almost certainly bound to Alpha Centauri, which is itself a binary star, moving with it through space in a stately celestial waltz.
Proxima has been the target of planet-search observations since the dawn of the exoplanet era. The new discovery used observations taken from the European Southern Observatory between 2000 and 2014, and an additional suite of data taken between January 19 and March 31 this year.
The technique used for the discovery is the Doppler wobble method, where observers measure the line-of-sight velocity of a star with exquisite precision, and watch for any evidence of the star rocking back and forth in space.
Introducing Proxima Centauri b
The evidence is strong that Proxima Centauri is host to at least one planet. The new world orbits the red dwarf star roughly every 11.2 days, at a distance of about seven million kilometres (far closer than Mercury orbits the sun).
Because the planet has been inferred from the line-of-sight wobble of its host, its mass remains uncertain, with the true value depending on the tilt of its orbit.
From the observations, we can tell that Proxima b is relatively tiny, with a minimum mass just 1.27 times that of the Earth, and this is where things get interesting.
Unlike the blazing furnace of the sun, Proxima is a tiny glowing ember. As such, a planet as distant from Proxima as the Earth is from the Sun would be frozen solid. But huddle close to the ember, and the temperature could be just right.
That’s why in theory, assuming Proxima b is truly Earth-like, it is in just the right place for liquid water to exist on its surface.
A truly habitable world?
Proxima b is about the right size and in about the right place, but could it really be habitable? Possibly, but it takes a lot more than the right kind of orbit to birth a habitable world.
Because Proxima b orbits so close to its parent star, it has almost certainly become tidally locked. This means Proxima b keeps one face perpetually pointed towards its star and bathed in daylight, with the other side facing away, in permanent night.
For many years, researchers thought that this would be a death knell for habitability, that the night-time side would be so cold that any atmosphere the planet had would freeze out onto its surface on that side, leaving it an airless, frozen husk.
But more recent climate models have suggested hope that with the right atmosphere, and the right weather, heat could be transported from day-side to night-side, keeping the warmth needed to stay habitable.
Would such a planet be truly habitable? We honestly don’t know – there’s just so much still to learn.
Megaflares and magnetic shields
Tidal locking isn’t the only hurdle that Proxima b faces in the race to habitability. Proxima is an active star, and experiences huge outbursts (“megaflares”) which could strip the atmosphere from any planet.
Once again though, this might not prove fatal. If the planet has a strong magnetic field, like the Earth, this would act as a magnetic shield, protecting the planet’s atmosphere from the worst vagaries of Proxima’s activity.
The mass of the planet suggests that, with an Earth-like composition, the planet could have a magnetic field.
With that extra mass comes extra radiogenic material, helping to keep the planet’s interior molten, which, in turn, could help to drive a dynamo creating the magnetic shield required to protect the atmosphere.
The race is certainly on to learn more about Proxima b. First, perhaps, will be observations that attempt to tie down the tilt of the planet’s orbit. Is it so perfectly aligned that it transits Proxima?
The odds aren’t great, but now we know the planet’s orbital period, we can predict when transits across its host star would be, and train our finest telescopes on the dim red dwarf to see whether it winks.
If it does, then the tilt of the planet’s orbit, and hence its mass, will become immediately apparent. Furthermore, such transits would allow astronomers to study Proxima b’s atmosphere – assuming it has one – through spectroscopic observations during the primary and secondary transits.
Far more likely, however, is that we will not see the planet transit. If it did, we’d probably have already have found it! In that case, all is not lost. In the coming years, it might be possible to directly image the planet, separating its light from that of its star.
Proxima’s proximity works in our favour, here. The planet’s orbital radius will place it, at most, 0.4 arcseconds from Proxima. That’s an angular separation equivalent to the width of an average human hair at a distance of about 75 metres.
A tiny gap, for sure, but one that is within the reach of instruments such as SPHERE, on the Very Large Telescope in Chile. The problem is the planet will be very, very faint compared to its host star.
If SPHERE can’t image Proxima b, then the proposed next generation of telescopes, either on Earth or in space, should be able to manage it.