It is far more common for stars to swallow at least one of their planets than previously recognized, a new study reveals. However, there is no need to fear our own Sun has imminent designs on Mercury, let alone Earth, since the new evidence relates to binary star systems (those with two stars).
Stars and planets are often referred to as if they had a familial relationship, with phrases such as “parental star”. To the extent that is true, it’s as much a replication of Roman myths about Saturn eating his children as what we think of as a family.
The fact stars sometimes consume planets has been known for a long time, but an attempt to quantify how often it happens, published in Nature Astronomy, produces a disturbingly high estimate to non-scientists' eyes. At the same time, the finding comes as something of a relief to astronomers, as it resolves a puzzle that has been bothering experts in stellar composition.
In theory, stars that form from the same gas cloud should have a similar abundance of elements. Yet many binary pairs contain one metal-rich and one metal-poor star, something the paper describes as “One of the most remarkable contradictions in modern stellar astrophysics.” In a tiny number of cases, these stars may have formed in different clouds and come to orbit each other later, but this would be too rare to notably affect the statistics.
Instead, two explanations were considered likely: either models are wrong and stars from the same cloud can start off with very different compositions, or it is common for one star to swallow a planet, changing its make-up in the process. The first would force us to throw out a lot of work based on the assumption of gas cloud homogeneity.
To distinguish between these possibilities Dr Lorenzo Spina of the Osservatorio Astronomico di Padova and co-authors noted stars with temperatures above 6,500°C (the Sun is around 6,000°C) keep a record of planets they swallow. Cooler stars, however, have thick convective outer layers, in which a planet's elements get mixed, diluting a typical planet to the point where its elements are undetectable. Hotter stars have much thinner shells, so the addition of a planet-worth of iron and silicon makes a noticeable difference,
The authors compared the frequency of visibly different compositions in pairs of hot stars and their cooler counterparts. They found differences are much more common in hot binaries, as would be expected if swallowing planets is common, but not if the stars started with differing elemental abundances.
Using this information they conclude 20-35 percent of stars in binary pairs have eaten at least one planet.
Young stars burn lithium rapidly, but this later slows down. Consequently, lithium abundance can provide some indication of the point at which the planet was swallowed. Spina told IFLScience the data makes clear not all the planet swallowing occurs in star systems' very early days, when objects zing around like pool balls. However, he added, we can't quantify yet how often planets are eaten late in life.
Although the studies were done on binary systems, Spina told IFLScience it's not safe to assume the disruptive effects of one star sends planets hurtling into the other in a way that wouldn't occur in a single star system, like ours. “Most of the binaries we looked at are very well separated. Disruption is probably only really an issue in tight binaries,” he said.