Debris spots found on stars reveal planets that went splat like bugs on a windshield.
The result: metal smears on the surface of parent stars, said European Southern Observatory astronomer Luca Pasquini, who offered up another analogy:
"It is a little bit like a tiramisu or a cappuccino," Pasquini said. "There is cocoa powder only on the top."
The finding could help unravel mysteries of planet formation.
As scientists began discovering exoplanets, or worlds orbiting distant stars, in the past decade, they found these planets were most often found around stars rich in iron.
In fact, stars that host planets are on average nearly twice as rich in metals than counterparts without worlds.
But are these stars rich in metals because planetary debris polluted them? Or do metal-loaded stars naturally spawn worlds? It's a classic chicken-or-egg problem.
If these metals were planetary debris, they would only be found in the outer layers of stars.
On the other hand, if these metals were inherently part of the stars, they would be found in their cores as well.
Unfortunately, the only light that astronomers can see from stars comes from their outermost layers, which means there is no direct way to peer into their hearts.
Instead, scientists looked at stars whose innards churn far more than our sun does. The ingredients of the interiors of these stars roil to their surfaces for astronomers to analyze.
Specifically, researchers focused on red giants, stars that — as will the sun in several billion years — have puffed up to become much larger and cooler after they have exhausted the hydrogen in their cores.
Compared with sun-like stars, these giants have much larger convective zones, or regions where all the gas is completely mixed.
The sun's convective zone comprises only 2 percent of the star's mass, but in red giants the convective zone is 35 times more massive.
After inspecting 14 planet-hosting red giants, Pasquini and his colleagues found these were not rich in metals as is typically the case for planet-hosting sun-like stars.
The simplest explanation is that metals seen in planet-hosting stars are pollution from planetary debris, findings that will be detailed in the journal Astronomy & Astrophysics.
The debris might come from "planets themselves or small planetoids," researcher Artie Hatzes, director of the Thuringia State Observatory in Tautenburg, Germany, told SPACE.com.
Pasquini said their results might favor the controversial and relatively new "disk instability" theory.
This concept states that large planets emerge from clumps of dust and gas whose hearts coalesce into cores that grow relatively quickly.
MIT planetary scientist Sara Seager, who did not participate in this study, noted "the findings are an intriguing piece of the puzzle in trying to understand planet formation."
Resolving the mysteries concerning how planets form will require a much larger study of metal-rich and metal-poor planet-hosting stars, she added.
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