Stars that feed on their planets carry a distinctive signature which can tell us just what they have been consuming. This will help us narrow the quest to find planets suitable for life.
The search for planets outside the solar system has revealed some that are not long for this, or any, world. Star-grazing planets, as they are called, have orbits so close to their parent star that they skim its outer edges. The heat sublimes the outer surface of the planet, just as cometary ice boils into space as they approach the sun. However, drag effects eventually see the planet get so close to the sun they are absorbed.
Since planets are proportionally far richer in elements heavier than helium (known to astronomers as metals) than stars absorption increases the star's metallicity. Metals have been observed in the outer layers of white dwarfs as a result of planet consumption. Graduate student Trey Mack of Vanderbilt University investigated what happens to sun-like stars in a similar scenario, publishing in The Astrophysical Journal.
"Trey has shown that we can actually model the chemical signature of a star in detail, element by element, and determine how that signature is changed by the ingestion of Earth-like planets,” says Mack's supervisor Professor Keivan Stassun. "After obtaining a high-resolution spectrum for a given star, we can actually detect that signature in detail, element by element."
A longstanding theory is that high metallicity stars are more likely to have planets. This makes sense since if the gas cloud from which a star formed had plenty of metals planets should find it easier to form. Population studies of stars observed for planets support this hypothesis. However, these studies have usually been based on a single measure of metallicity – the ratio of iron to hydrogen.
Mack instead looked at the concentrations of 15 elements in the sun and a pair of comparison stars. He included the elements considered most important to the formation of Earth-like planets: aluminum, silicon, calcium and iron itself.
The stars chosen, HD 20781 and 20782, are so far unique in being a wide binary pair, both of which are known to have planets in orbit. Both stars are G-type, like the sun, although they are substantially older. While the stars are similar, their known planets are not. HD 20781 has two planets of mass similar to Neptune, one with an orbit similar to Mercury, the other much closer in. On the other hand HD 20782 is orbited by a planet almost twice the mass of Jupiter with an orbit that ranges from closer than Mercury to further out than Mars. Until 2012 this was the most elongated (“eccentric” to astronomers) orbit known.
Both of these stars had slightly higher concentrations of the key elements studied than the sun, despite having been born at a time when the galaxy as a whole had a lower metal content than at the birth of the sun. More intriguingly, two stars thought to have formed from the same gas cloud have different concentrations of metals from each other.
"Imagine that the star originally formed rocky planets like Earth. Furthermore, imagine that it also formed gas giant planets like Jupiter," says Mack. "The rocky planets form in the region close to the star where it is hot and the gas giants form in the outer part of the planetary system where it is cold. However, once the gas giants are fully formed, they begin to migrate inward and, as they do, their gravity begins to pull and tug on the inner rocky planets. With the right amount of pulling and tugging, a gas giant can easily force a rocky planet to plunge into the star. If enough rocky planets fall into the star, they will stamp it with a particular chemical signature that we can detect."
Mack found that the higher the melting point of an element, the higher its concentration in the pair, relative to the sun. The measurements are exactly the pattern that would be expected if 20782 had started out with rocky planets with combined masses 10 times that of the Earth, which have since been absorbed, while 20781 had swallowed 20 Earth masses.
Unsurprisingly, Mack and Stassun doubt either star still hosts Earth-like planets. If the findings are found to apply more widely, Mack says, “When we find stars with similar chemical signatures, we will be able to conclude that their planetary systems must be very different from our own and that they most likely lack inner rocky planets.” The search for planets that might host life can then focus on stars with concentrations of high melting point metals similar to the sun.
Stassun says, "This work reveals that the question of whether and how stars form planets is actually the wrong thing to ask. The real question seems to be how many of the planets that a star makes avoid the fate of being eaten by their parent star?"
Stassun and Mack discuss the results below.