What is the rarity of our solar system? We’ve learned that planetary systems are widespread throughout the Galaxy in the 30 years or so since planets were first identified circling stars other than our Sun. Many of them, though, are very different from the Solar System we are familiar with.
Our Solar System’s planets orbit the Sun on stable, almost circular trajectories, implying that their orbits haven’t altered significantly since the planets were formed. However, many planetary systems circling other stars have had a tumultuous history.
Our Solar System’s relatively tranquil past has encouraged the growth of life on Earth. If we can find a means to identify systems that have had comparable tranquil pasts, we can limit down our search for extraterrestrial worlds that may contain life.
In a study published in Nature Astronomy, our multinational team of astronomers took on this problem. We discovered that between 20% and 35% of Sun-like stars consume their own planets, with the most plausible figure being 27%.
This implies that at least a quarter of planetary systems orbiting stars like the Sun had a chaotic and dynamic past.
Chaotic histories and binary stars
Exoplanetary systems with big or medium-sized planets have been seen by astronomers in numerous cases. Other planets’ trajectories may have been altered or even forced into unstable orbits by the gravitational pull of these migrating planets.
Some planets are believed to have fallen into the host star in most of these highly active systems. We didn’t know, however, how frequent these chaotic systems are in comparison to more ordered systems like ours, which have encouraged the flourishing of life on Earth due to their organized construction.
Directly studying exoplanetary systems would be extremely difficult, even with the most precise astronomical instruments available. Instead, we looked at the chemical makeup of binary star systems.
A binary system is made up of two stars orbiting each other. We anticipate the two stars to have the same combination of elements because they originated from the same gas at the same time.
A planet that collides with one of the two stars, on the other hand, is dissolved in the star’s outer layer. This can change the star’s chemical composition, allowing us to see more of the elements that make up rocky planets, such as iron, than we would otherwise.
Traces of rocky planets
By studying the spectrum of light produced by 107 binary systems made up of Sun-like stars, we were able to examine their chemical makeup. As a result, we were able to determine how many stars had more planetary material than their companion star.
We also discovered three things that add up to unmistakable proof that the chemical differences between binary pairs were caused by planets eating each other.
First, we discovered that stars with a thinner outer layer are more likely to be iron-rich than their companions. This is compatible with planet-eating, because diluting planetary material in a thinner out layer causes a larger change in the chemical composition of the layer.
Second, stars with higher levels of iron and other rocky-planet elements have more lithium than their counterparts. In stars, lithium is quickly depleted, yet it is preserved on planets. As a result, an abnormally high quantity of lithium in a star must have arrived after the star formed, which corresponds with the theory that lithium was transported by a planet until it was devoured by the star.
Third, stars with more iron than their companions have more iron than similar stars elsewhere in the Galaxy. The same stars, on the other hand, contain standard abundances of carbon, a volatile element that is not transported by rocks. As a result, pebbles from planets or planetary material have chemically enriched these stars.