NASHVILLE - The discovery of the three smallest planets yet orbiting a
distant star, which was announced last week at the annual meeting of the
American Astronomical Society, has an unusual connection to Barnard’s
star, one of the Sun’s nearest neighbors.
The discovery was made
by a scientific team led by astronomers at the California Institute of
Technology (Caltech) that included three members from Vanderbilt. The
team used data from NASA’s Kepler mission combined with additional
observations of a single star, called KOI-961, to determine that it
possesses three planets that range in size from 0.57 to 0.78 times the
radius of Earth. This makes them the smallest of the more than 700
exoplanets confirmed to orbit other stars.
In their investigation
of KOI-961, which is about 130 light years away in the Cygnus
constellation, the astronomers found that it is nearly identical to
Barnard’s star, which is only six light years away in the constellation
Ophiuchus. This similarity allowed them to use information about
Barnard’s star, which was discovered in 1916 by Vanderbilt astronomer
E.E. Barnard, to determine the mass, size and luminosity of the distant
star. These values, in turn, were used to determine the size of the
three new exoplanets.
“Barnard’s star and KOI-961 are both M
dwarfs, which are also known as red dwarfs. This is the smallest
category of stars. They are popular targets for exoplanet hunters
because their small size makes it easier to detect Earth-sized planets,”
said Keivan Stassun, the professor of astronomy who headed the
Vanderbilt contingent. The other Vanderbilt scientists involved were
Research Assistant Professors Joshua Pepper and Leslie Hebb.
the 1960’s through the 1980’s, astronomers thought that Barnard’s star
also had a planetary system – specifically one or two planets larger
than Jupiter. If their existence had been verified, it would have been a
scientific first, but the evidence was ultimately discredited. Today,
advances in telescope technology and image processing allow astronomers
to identify stars with exoplanets with considerable confidence.
Barnard’s star favorite of science fiction destination
Barnard’s star is too dim to be seen by the naked eye, its proximity to
the Sun and the possibility that it possessed a planetary system made
it a favorite destination for science fiction writers. It appears in
dozens of science fiction novels, including Hitchhiker’s Guide to the
Galaxy, movies like the 1979 film The Alien Encounters, television
series including Galactica Discovers Earth and a number of computer and
By contrast, KOI-961 is one of thousands of nameless
stars that NASA’s Kepler mission has identified as candidates that may
possess planetary systems. The Kepler spacecraft contains a specially
designed telescope that continuously monitors the brightness of 150,000
stars at a time. It flags stars whose brightness dips periodically
because the dimming could be caused by a planet that passes in the front
of the star as viewed from Earth. Astronomers call this the transit
method of planet detection.
The Caltech team used the Kepler data
on KOI-961 along with follow-up observations from the Palomar
Observatory near San Diego and the W.W. Keck Observatory in Hawaii to
confirm the existence of its planetary system and to determine the size
of its planets.
Vanderbilt astronomers helped determine star’s size
transit method provides astronomers with the ratio of the size of the
planet to that of the star. As a result, they needed to determine the
star’s size to calculate the size of the planets. The Kepler telescope
gives some crude information about a star’s diameter, but the
researchers knew that this data is particularly unreliable for M dwarfs,
Stassun said. So the Vanderbilt contingent performed the additional
telescope observations and analysis that were required to get an
accurate estimate of the star’s size.
To get better estimates of the star’s properties, the astronomers
obtained an accurate measure of the star’s color from Vanderbilt’s
telescope in southern Arizona and a detailed spectrum of the star from
Palomar and Keck. This provided a fingerprint of KOI-961. “When we
compared its fingerprint with those of the best known M dwarfs we found
that Barnard’s star was the best match,” said Stassun.
That was fortunate because Barnard’s star is the one of the most studied
and best characterized M dwarfs. Specifically, there is an accurate
estimate of its size, which is one-fifth that of the Sun. This allowed
the researchers to start with a mathematical model of Barnard’s star and
alter it to account for the subtle differences between the two stars.
When they did, the model produced an even smaller estimate of KOI-961’s
size: about one-sixth that of the Sun.
Once the size of the star was established, the team used the Kepler data
to calculate that the three exoplanets range from the size of Mars to
slightly more than three-quarters the size of Earth. They also
determined that these planets orbit the star with periods ranging from a
half day to two days. Such short periods mean that all three orbit so
close to their star that they must be too hot for liquid water to exist
and life to evolve, the astronomers calculate.
New system comparable in size to Jupiter and its moons
The diminutive dimensions of this planetary system prompted John
Johnson, the principal investigator of the research from NASA's
Exoplanet Science Institute at Caltech, to comment, "The really amazing
thing about this system is that the closest size comparison is to
Jupiter and its moons." (KOI-961 is just 70 percent bigger than Jupiter
and its exoplanets are comparable in size and have similar orbital
periods to the Galilean moons that circle the Jovian planet.)
The fact that Barnard’s star doesn’t have a giant planet doesn’t
preclude the possibility that it has smaller planets. The discovery of
another M dwarf that has small exoplanets increases the likelihood that
Barnard’s star may have some as well. If it does, however, the planets
must orbit at a much greater distance than those at KOI-961. The Kepler
mission requires that the image of a star must dip three times before it
is tagged as a planet-bearing candidate. As a result, the longer a
planet’s orbital period, the more difficult it is to discover. For
example, if a planet orbits a star once a year, it would take three
years of continuous observations to detect in this fashion.
The Vanderbilt team’s contribution was supported by Vanderbilt’s Initiative in Data-intensive Astrophysics.
This article was first published at www.newswise.com