In the star cluster NGC 6811, astronomers have found two planets smaller than Neptune orbiting Sun-like stars.
Credit: Michael Bachofner. High Resolution Image (jpg) - Low Resolution Image (jpg)
Cambridge, MA - All
stars begin their lives in groups. Most stars, including our Sun, are
born in small, benign groups that quickly fall apart. Others form in
huge, dense swarms that survive for billions of years as stellar
clusters. Within such rich and dense clusters, stars jostle for room
with thousands of neighbors while strong radiation and harsh stellar
winds scour interstellar space, stripping planet-forming materials from
nearby stars.
It would thus seem an unlikely place to find alien worlds. Yet 3,000
light-years from Earth, in the star cluster NGC 6811, astronomers have
found two planets smaller than Neptune orbiting Sun-like stars. The
discovery, published in the journal Nature, shows that planets can develop even in crowded clusters jam-packed with stars.
"Old clusters represent a stellar environment much different than the
birthplace of the Sun and other planet-hosting field stars," says lead
author Soren Meibom of the Harvard-Smithsonian Center for Astrophysics
(CfA). "And we thought maybe planets couldn't easily form and survive in
the stressful environments of dense clusters, in part because for a
long time we couldn't find them."
The two new alien worlds appeared in data from NASA's Kepler spacecraft.
Kepler hunts for planets that transit, or cross in front of, their host
stars. During a transit, the star dims by an amount that depends on the
size of the planet, allowing the size to be determined. Kepler-66b and
Kepler-67b are both less than three times the size of Earth, or about
three-fourths the size of Neptune (mini-Neptunes).
Of the more than 850 known planets beyond our solar system, only four -
all similar to or greater than Jupiter in mass - were found in clusters.
Kepler-66b and -67b are the smallest planets to be found in a star
cluster, and the first cluster planets seen to transit their host stars,
which enables the measurement of their sizes.
Meibom and his colleagues have measured the age of NGC 6811 to be one
billion years. Kepler-66b and Kepler-67b therefore join a small group of
planets with precisely determined ages, distances, and sizes.
Considering the number of stars observed by Kepler in NGC 6811, the
detection of two such planets implies that the frequency and properties
of planets in open clusters are consistent with those of planets around
field stars (stars not within a cluster or association) in the Milky Way
galaxy.
"These planets are cosmic extremophiles," says Meibom. "Finding them
shows that small planets can form and survive for at least a billion
years, even in a chaotic and hostile environment."
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.
Soren Meibom
Astronomer
smeibom@cfa.harvard.edu
smeibom@gmail.com
David A. Aguilar
Director of Public Affairs
Harvard-Smithsonian Center for Astrophysics
617-495-7462
daguilar@cfa.harvard.edu
Christine Pulliam
Public Affairs Specialist
Harvard-Smithsonian Center for Astrophysics
617-495-7463
cpulliam@cfa.harvard.edu
