A planet several times more massive than Earth may have disappeared into the Sun, leaving traces that can still be detected today.
The scenario stems from an old observation: models describing the Sun’s interior do not perfectly reproduce certain real measurements. The vibrations of our star make it possible, in particular, to deduce how sound travels beneath its surface. Yet the calculated values differ from these observations, especially beneath the outer layer where matter circulates.
Another clue puzzles astronomers. The Sun’s surface contains far less lithium than expected based on the material present when it formed. Mutlu Yıldız, of Ege University in Turkey, therefore investigated whether these two anomalies could have a common origin dating back to the Sun’s youth.
The researcher simulated the evolution of our star by adding an unusual event: the absorption of a planet rich in heavy elements. The models that best match the observations involve a super-Earth of about 5 to 10 Earth masses. One of the best fits gives a mass close to 5.6 times that of Earth.
In concrete terms, this planet’s material would not simply have disappeared into the surface layers. Some of it may have sunk below the zone where solar matter mixes continuously. It would then have locally enriched the Sun’s interior with heavy elements. This change slightly alters the way energy and sound travel through the star.
The calculations indicate that a sufficiently compact rocky world could pass through the young Sun’s outer layers without immediately losing all its material. Its contents could therefore reach deeper regions before dispersing.
Artist’s view of a planet skimming and then plunging into its star’s envelope.
Credit: K. Miller/R. Hurt (Caltech/IPAC), MIT.
The scenario can also reproduce the Sun’s low present-day lithium abundance. In Mutlu Yıldız’s study, the young Sun would initially have had a slightly lower mass, then incorporated several Earth masses of material. To reproduce the current abundance, this added material must have been very poor in lithium. It would then increase the Sun’s mass and alter its composition without adding a proportionate amount of lithium. The lithium already present would therefore become more diluted and subject to internal mixing.
This is rather counterintuitive because an ordinary rocky planet could instead supply lithium and temporarily enrich a star’s surface. Indeed, many planetary engulfment models predict exactly this.
The scenario in this new study therefore requires a particular condition: the swallowed super-Earth must already have been severely depleted in lithium. The author finds that with about 4.6 to 5.8 Earth masses of lithium-poor material, the model can reproduce the current solar abundance.
This hypothesis also offers a possible answer to a peculiarity of our neighborhood. Many planetary systems possess super-Earths close to their star, whereas the Solar System has none. Earlier studies had already proposed that such planets could have formed near the Sun before migrating toward it.
The researcher compared this scenario with other possible changes to solar models, particularly those involving the transport and mixing of matter. These solutions improve certain results, but absorbing a planet provides a better simultaneous agreement with several measurements in the simulations studied.
The next step will therefore be observational. If the predicted enrichment in heavy elements beneath the Sun’s outer layers has a measurable signature, the vibrations of our star could make it possible to search for it. Such a detection would constitute independent evidence of an event that occurred several billion years ago.