In
this artist's conception, a carbon planet orbits a sunlike star in the
early universe. Young planetary systems lacking heavy chemical elements
but relatively rich in carbon could form worlds made of graphite,
carbides and diamond rather than Earth-like silicate rocks. Blue patches
show where water has pooled on the planet's surface, forming potential
habitats for alien life. Christine Pulliam (CfA). Sun image: NASA/SDO. High Resolution (jpg) - Low Resolution (jpg)
Cambridge, MA -Our
Earth consists of silicate rocks and an iron core with a thin veneer of
water and life. But the first potentially habitable worlds to form
might have been very different. New research suggests that planet
formation in the early universe might have created carbon planets
consisting of graphite, carbides, and diamond. Astronomers might find
these diamond worlds by searching a rare class of stars.
"This work shows that even stars with a tiny fraction of the carbon in our solar system can host planets," says lead author and Harvard University graduate student Natalie Mashian.
"We have good reason to believe that alien life will be carbon-based,
like life on Earth, so this also bodes well for the possibility of life
in the early universe," she adds.
The primordial universe consisted mostly of hydrogen and helium, and
lacked chemical elements like carbon and oxygen necessary for life as we
know it. Only after the first stars exploded as supernovae and seeded
the second generation did planet formation and life become possible.
Mashian and her PhD thesis advisor Avi Loeb (Harvard-Smithsonian
Center for Astrophysics) examined a particular class of old stars known
as carbon-enhanced metal-poor stars, or CEMP stars. These anemic stars
contain only one hundred-thousandth as much iron as our Sun, meaning
they formed before interstellar space had been widely seeded with heavy
elements.
"These stars are fossils from the young universe," explains Loeb. "By
studying them, we can look at how planets, and possibly life in the
universe, got started."
Although lacking in iron and other heavy elements compared to our
Sun, CEMP stars have more carbon than would be expected given their age.
This relative abundance would influence planet formation as fluffy
carbon dust grains clump together to form tar-black worlds.
From a distance, these carbon planets would be difficult to tell
apart from more Earth-like worlds. Their masses and physical sizes would
be similar. Astronomers would have to examine their atmospheres for
signs of their true nature. Gases like carbon monoxide and methane would
envelop these unusual worlds.
Mashian and Loeb argue that a dedicated search for planets around
CEMP stars can be done using the transit technique. "This is a
practical method for finding out how early planets may have formed in
the infant universe," says Loeb.
"We'll never know if they exist unless we look," adds Mashian.
This research has been accepted for publication in the Monthly Notices of the Royal Astronomical Society and is available online.
Headquartered
in Cambridge, Mass., the Harvard-Smithsonian Center for Astrophysics
(CfA) is a joint collaboration between the Smithsonian Astrophysical
Observatory and the Harvard College Observatory.
CfA scientists,
organized into six research divisions, study the origin, evolution and
ultimate fate of the universe.
For more information, contact:
Christine Pulliam
Media Relations Manager
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
