Astronomers Seize Rare Opportunity To Weigh Sun’s Nearest Neighboring Star

It takes a telescope like the Hubble to make Proxima Centauri look bright, but an unusual event to know how just how massive this small star is. ESA/Hubble & NASA

The discovery of a probable Earth-like planet around Proxima Centauri put a new focus on Earth’s nearest star, the Sun, aside. Astronomers have seized a rare opportunity to refine our estimates of its mass, and therefore of the Earth-like planet that orbits it.

The mass of a star is possibly its most important metric. We can measure the mass of stars that orbit each other precisely, using their orbital period and Newton’s laws. For solo stars, we generally have to extrapolate from brightness and similarities to stars we can measure.

Although technically Proxima Centauri orbits Alpha Centauri A and B, it’s so distant, and so much lighter than that pair, it’s gravitational influence is negligible. Dissatisfied with relying on comparisons with similar stars of known mass, Dr Alice Zurlo of Universidad Diego Portales, Chile, used a rare case of Proxima passing between us and a distant star to do better.

When massive objects pass in front of a light source they bend the radiation, creating a gravitational lens. The greater their mass, the stronger the bending.

Stars with a rich starfield behind them provide frequent opportunities for what are called microlensing events, but Proxima is less fortunate. Nevertheless, in 2014 and 2016 its movement relative to two distant stars was suitable for microlensing measurements. The first opportunity was missed, but Zurlo took full advantage of the second.

In a paper accepted for the Monthly Notices of the Royal Astronomical Society Zurlo reports Proxima’s mass is 0.15 times that of the Sun, with an error of less than 40 percent. Previous estimates were 0.12 solar masses. The finding raises the minimum possible mass for Proxima b to 1.5 times the Earth’s mass.

A star other than the Sun was first weighed this way only last year. Proxima will not pass a star this bright again for 20 years.

 As a small star, so faint that even backyard telescopes can’t spot it despite its closeness, Proxima was long overlooked. That all changed when small wobbles in its location alerted astronomers to the probable presence of a planet circling it – one with a mass possibly just 30 percent greater than the Earth’s and at a suitable distance for a mild climate.

This year an enormous stellar flare erupted from Proxima, blasting any nearby planets with so much radiation that prospects for life look grim. Still, even if any planets around Proxima can’t support life, they’re still among the best opportunities we will have to study planets beyond our Solar System in depth

People are even modeling the crew size needed for a mission to Proxima b. The subsequent discovery of dust belts around Proxima created an extra reason to be interested.

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