Astronomers at the Harvard-Smithsonian Center for Astrophysics have
discovered a planet somewhat larger and more massive than Neptune
orbiting a star 120 light-years from Earth. While Neptune has a diameter
3.8 times that of Earth and a mass 17 times Earth’s, the new world
(named HAT-P-11b) is 4.7 times the size of Earth and has 25 Earth
HAT-P-11b was discovered because it passes directly in front of
(transits) its parent star, thereby blocking about 0.4 percent of the
This periodic dimming was detected by a network of small, automated
telescopes known as “HATNet,” which is operated by the Center in Arizona
and Hawaii. HAT-P-11b is the 11th extrasolar planet found by HATNet, and
the smallest yet discovered by any of the several transit search
projects underway around the world.
Transit detections are particularly useful because the amount of dimming
tells the astronomers how big the planet must be. By combining transit
data with measurements of the star’s “wobble” (radial velocity) made by
large telescopes like Keck, astronomers can determine the mass of the
A number of Neptune-like planets have been found recently by radial
velocity searches, but HAT-P-11b is only the second Neptune-like planet
found to transit its star, thus permitting the precise determination of
its mass and radius.
The new found world orbits very close to its star, revolving once every
4.88 days. As a result, it is baked to a temperature of around 1100
The star itself is about three-fourths the size of our Sun and somewhat
There are signs of a second planet in the HAT-P-11 system, but more
radial velocity data are needed to confirm that and determine its
Another team has located one other transiting super-Neptune, known as
GJ436b, around a different star. It was discovered by a radial velocity
search and later found to have transits.
“Having two such objects to compare helps astronomers to test theories
of planetary structure and formation,” said Harvard astronomer Gaspar
Bakos, who led the discovery team.
HAT-P-11 is in the constellation Cygnus, which puts in it the field of
view of NASA‘s upcoming Kepler spacecraft. Kepler will search for
extrasolar planets using the same transit technique pioneered by
ground-based telescopes. This mission potentially could detect the first
Earth-like world orbiting a distant star. “In addition, however, we
expect Kepler to measure the detailed properties of HAT-P-11 with the
extraordinary precision possible only from space,” said Robert Noyes,
another member of the discovery team.