I think it’s exciting that we’ve found an Earth-sized planet orbiting the star right next door. Proxima Centauri – the closest star to us besides the Sun.
Last week’s announcement of Proxima b by the ‘Pale Red Dot’ project, operating mainly from the European Southern Observatory in Chile, was ‘leaked’ a month or so ago. Now it’s been announced properly, with an accompanying paper in the August edition of Nature. And outside the science community there’s been the usual flood of supposition about aliens, Earth’s twin, and the rest. We want to find another Earth, aliens to talk to, it seems. New discoveries – any discoveries – are conceptualised along those lines even when there’s no evidence to support the idea. According to one anthropological study, even astronomers sometimes get pulled into that one.
In fact, so far, there’s little data on the new planet. It hasn’t been imaged – and current telescopes can’t do so. It was discovered by analysing Proxima’s ‘wobble’ as the planet orbited, using doppler techniques – light ‘blued’ by a microscopic amount as Proxima was pulled towards us and ‘reddened’ similarly as it was pulled away. Although the fidelity needed to pick up those miniscule changes has been around in instruments for a decade, it’s still been an amazing technical achievement because Proxima is prone to flaring – which mimics the light-curve change, but isn’t due to a planet. That had to be analysed and eliminated first. The data also suggested a possible other planet in the Proxima system, but there’s no confirmation yet. I’d be surprised if the confirmed planet was the only one, though.
Unfortunately, all we know about the planet just now is the period – 11.2 days; the radius of the orbit – 7.5 million kilometres; and mass, which at 7.76 x 10<exp>24 kg is 1.3 times Earth. That implies it’s a rocky world, like Earth, but we can’t say for sure – ESA’s artists’ impression is a possibility, but still speculation. Because Proxima’s luminosity is known, we can derive the surface temperature. That turns out to be -40 degrees C in vacuum. For comparison, Earth’s equivalent is -20 degrees C, but we have an atmosphere, which warms us up a bit. Proxima b might too, implying it’s warmer on the surface. Nobody knows. It’s in the ‘habitable zone’, which is where water can be liquid – but we don’t know if it has water. There’s every chance it’s tidally locked – presenting the same face to Proxima like the Moon does to Earth.
Proxima Centauri is the closest star to the Sun, and will be until Barnard’s Star nears its 3.8 light year closest approach in about 9700 years. Close, of course, is a relative term: it’s 4.24 light-years distant. If the Sun was the size of a full stop – 0.5 mm – Earth would be 4.65 cm from it (unless I’ve dropped a decimal point) – and Proxima some 13.8 kilometres distant. Woah! Proxima is invisible to the naked eye – an M5.5Ve class red dwarf with 0.12 times the mass of the Sun and 0.0017 times its luminosity. It’s only about half again as massive as Jupiter, and is at the bottom end of the range of objects that can be a proper star.
It might be part of the larger Centauri system, the double- or triple-star system that is our nearest visible neighbour, as long as you’re not more than 29 degrees north. I’m in the southern hemisphere, which means I can stick my head out the window and spot Alpha Centauri against the glare of the streetlights. It’s one of the ‘pointer’ stars to Crux, the Southern Cross. The two stars appear as one to the naked eye and (by a smidgeon) are jointly third-brightest in the sky. The system comprises a G2v-class star (Alpha Centauri A) which is a near-twin of the Sun, and a smaller K2v-class orange dwarf (Alpha Centauri B) orbiting a common centre of gravity (barycentre) in ways that bring them as close as 11 times the Earth-Sun distance and out to more than 60 times that distance. There are probably planets, though a 2012 discovery around Alpha Centauri B was a false alarm. Whether Proxima is part of the mix is moot. It’s 0.2 of a light year from the other two – 15,000 times the Earth-Sun distance – and may or may not be gravitationally bound to them.
The main problem for a planet there is that Proxima is a flare star. Quite often – sometimes just a few hours apart – this cool red dwarf erupts with a fury enough to blowtorch nearby space with high doses of radiation. If Earth was orbiting Proxima, instead of the newly discovered planet, our magnetic field would protect us to some extent. But if we were in the way of a coronal mass ejection we’d suffer a ‘super-Carrington’ electrical storm – and the chances are very high of that happening a lot, given the frequency of flares. Eventually Earth would be blasted to a lifeless wasteland, its atmosphere stripped away. Though chefs might do a good trade in crème brulee along the way.
Needless to say, there’ll be major studies of the new planet. For now, they’ll be limited to ground-based and near-Earth space-based instruments. Could we send a probe? Absolutely. But remember that distance – Proxima might be the closest star besides the Sun, but it’s still more than 8000 times further away than Pluto – and it took New Horizons, the fastest object ever made by humanity, 11 years just to get to Pluto.
There’s a project under way to build interstellar micro-probes the size of a postage stamp that could get there more quickly, towed by light-sails driven by Earth-based laser beams. Nobody knows exactly how yet, though. One challenge, apparently, is stopping them being eroded by interstellar particles as they plough through space at 20 percent the speed of light. I suspect there could also be adventures getting them to ‘phone home’ using the kind of transmitters you can fit on a micro-chip. So it’s not a quick project – 20-30 years to raise the funds and build the tech, another 20-25 years to fly there, and 4.24 years for the signals to come back. I bet that for all the technical challenges, the real hurdle will be funding.
Meanwhile, it’s all eyes on Proxima. I won’t speculate as to what the planet might be like – but if you want to check out my fictional take on other star systems and aliens, it’s in my novella ‘Missionary’.
Copyright © Matthew Wright 2016