Measuring the mass of Proxima Centauri
Before you can calculate the weight of a planet, you first have to calculate the weight of its star
R ecently, I’ve spent a lot of time thinking about how little we really know about the Universe. The broad picture – how galaxies and stars and planets form and evolve – seems clear, but the nitty gritty details are more difficult to pin down. Even when it comes to the objects closest to us, we still know less than we’d like.
Take Proxima Centauri, for example. The nearest star to the Solar System, it’s a faint red dwarf and companion to the much brighter twin stars of Alpha Centauri. It’s right there – a little over four lightyears away – and yet until recently we didn’t have a good idea of how massive it was.
Understanding the star’s mass became more than a curiosity when a planet was discovered in orbit around Proxima at a distance that might make it a habitable world. If we know the mass of the planet, we can work out whether it might be rocky. But since the planet has only been detected indirectly by observing the pull of its gravity on the star itself, you can’t know the properties of the planet better than the properties of the star.
Luckily, in February 2016 Proxima passed close to a more distant star, allowing an international team to make use of a technique called microlensing. The light from the distant source is bent by the gravity of Proxima, in a way precisely predicted by Einstein’s relativity; the effect is a small scale version of what happens in the spectacular gravitational lenses that reveal distant galaxies.
Blind searches for microlensing events – where the instrument monitors the brightness of many tens or even hundreds of thousands of stars hoping to catch the odd microlensing event – have been used before to hunt for dark matter and freefloating planets, but this one was predicted.
That’s just as well, because it was the opposite of spectacular. Using the SPHERE instrument on the Very Large Telescope in Chile, the researchers saw the background star shift by not much more than a milliarcsecond, which is about the size of a pound coin in Edinburgh as seen from London. That degree of precision requires careful calibration and processing of data, but the results are worth it.
The mass of Proxima is, it turns out, 15 per cent that of the Sun (with an error margin of around 40 per cent). This is larger than had been calculated by studying the star alone, and that has consequences for the planet, whose minimum mass is now at least one and a half times that of Earth. This is getting to the point where we should imagine not a large Earth, but a mini-Neptune. There is good news for planet fans, though.
A single transit, possibly the result of a second planet in the system, was spotted in August 2016. But as no second transit has been seen no one can confirm if it was due to a background binary star rather than a planet. However, if there were a background binary in the system it should have revealed itself in the microlensing event. It didn’t. Now we know the star’s mass, the Proxima system may have more surprises in store.
“The background shifted by not much more than a milliarcsecond, which is about the size of a pound coin in Edinburgh as seen from London”