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

Sky at Night Magazine - - BULLETIN - CHRIS LIN­TOTT was read­ing… The grav­i­ta­tional mass of Prox­ima Cen­tauri mea­sured with SPHERE from a mi­crolens­ing event by A Zurlo, et al. Read it on­line at

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”

Thanks to an in­fin­tes­i­mally tiny wob­ble we now know the mass of Prox­ima Cen­tauri (im­aged here by Hub­ble)

CHRIS LIN­TOTT is an as­tro­physi­cist and co-pre­sen­ter of The Skyat Night on BBC TV. He is also the di­rec­tor of the Zooni­verse project

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