Relationships can be complicated. The relationships between stars and their orbiting planets are especially so.
Planets are essentially leftovers. They form the material that’s left after the creation of their host star. Their entire existence is, in turn, molded by that star.
Thankfully this hasn’t happened yet in our own star system. Understanding why some stars do engulf planets and how this engulfment influences their chemical makeup provides an essential clue: These are the fingerprints scientists can use to find “Solar System analogs” and exclude star systems inhospitable to life.
“We now have a potential ‘upstream’ method to identify those Sun-like stars that are less likely to host Earth-like planets, which could be useful as a criterion for planet searches,” the study team writes.
The study was published Monday in the journal Nature Astronomy.
What you need to know first — Each star system is unique.
Our Sun was born 4.6 billion years ago, and the remaining material swirled into an accretion disc around the young star. From this cosmic soup of gas, dust, and rock, the planets of the Solar System were born.
The Solar System features rocky planets like Earth, gas giants like Jupiter, and icy worlds like Neptune. Each star system in the universe features a different set of planets, suggesting that the history of each star and its orbiting planets are diverse. This diversity extends to how a star interacts with planets.
Research suggests there are chemical differences across Sun-like stars. Why this is has been primarily explained by two theories: differences in protostellar gas clouds or “planet engulfment” events. The study team writes:
“The former scenario undermines the general belief that the chemical makeup of stars provides the fossil information of the environment in which they formed, whereas the second scenario would shed light on the possible evolutionary paths of planetary systems.”
This study supports the theory of planetary engulfment and helps explain why the Sun has an “unusual and still unexplained chemical composition when compared to other Sun-like stars” — it hasn’t swallowed up its planets.
What’s new— Planetary engulfment scenario happens around Sun-like stars: Their orbiting planets grow unstable and plunge into the fiery hearts of the stars. This study is the most detailed look at this scenario yet, providing an estimate of how likely this is to occur.
Stars are made up of light materials — like hydrogen, helium, oxygen, and carbon — while rocky planets like Earth are rich in heavy elements like iron, silicon, and titanium. When one of those planets is swallowed up by its star, the heavy elements from the planet leaves a strong signature on the star’s outer layers which can be observed in its light.
The study team argues this suggests that, if a star is shown to be heavy in iron, then that could likely be due to it snacking on one of its planets.
To get to these conclusions, the researchers examined 107 binary star systems with two Sun-like stars. Thirty-three of the systems revealed stars heavy in iron: This is a sign that a planet was swallowed.
From their observations, the team concluded that between 20 and 35 percent of Sun-like stars engulf some of their planets.
This may have occurred when the gravitational tug between two planets around the star caused one of them to fly straight into the star’s center, or push it close enough for the radiation from the star to slowly vaporize the planet and serve it as a hot lunch.
The study also reaffirms that our Sun is no such criminal: Our host star doesn’t contain the host of heavy elements found across the cannibal stars.
Why this matters — This research is a reminder of how unique our Sun really is. It is a yellow dwarf, formally known as a G-type star, which makes up around seven percent of all stars in the Milky Way.
Most stars of this kind are born in pairs. And while some astronomers believe that the Sun was actually born as part of a binary-star system, meaning it had a twin at time of birth, the star is on its own today and has been for most of its lifetime.
As far as we know, the Sun is the “only Sun-like star that we know is hosting an Earth-like planet,” the study team writes. Its unique chemical composition is indicative of a relatively calm star system: One where planets are safe, for now, from engulfment.
This suggests this research could help astronomers in their search for habitable planets in the universe.
When searching for habitability, astronomers look for planets that are similar to Earth and therefore have a better chance of hosting life. By looking at the elements of a star, astronomers can look to see if such a star is heavy on iron or other material, suggesting its Earth-like planet is gone.
This can help them cross certain star systems off the list: If a star is swallowing planets, it’s probably not the best place for scientists to hunt for evidence of life.
Abstract: Stellar members of binary systems are formed from the same material, and therefore they should be chemically identical. However, recent studies have unveiled chemical differences between the two members of binary pairs composed of Sun-like stars. These chemically inhomogeneous binaries represent one of the most contradictory examples in stellar astrophysics and a source of tension between theory and observations. It is still unclear whether the abundance variations are the result of inhomogeneities in the protostellar gas clouds or are due to planet engulfment events that occurred after the stellar formation. The former scenario undermines the general belief that the chemical makeup of stars provides the fossil information of the environment in which they formed, whereas the second scenario would shed light on the possible evolutionary paths of planetary systems. Our study provides compelling evidence in favor of the planet engulfment scenario. We also establish that planet engulfment events occur in Sun-like stars with a 20–35% probability. Therefore, an important fraction of planetary systems undergo very dynamical evolutionary paths that critically modify their architectures, unlike our calm Solar System. This study opens the possibility of using chemical abundances of stars to identify which ones are the most likely to host Solar System analogs.