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. 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 to their cores. Unfortunately, the only light that
astronomers can see from stars comes just 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 and 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.
advertisement
