(August 4, 2015) Skygazers
at northern latitudes are familiar with the W-shaped star pattern of Cassiopeia
the Queen. This circumpolar constellation is visible year-round near the North
Star. Tucked next to one leg of the W lies a modest 5th-magnitude star named HD
219134 that has been hiding a secret.
Astronomers have now teased out that secret: a planet in a
3-day orbit that transits, or crosses in front of its star. At a distance of
just 21 light-years, it is by far the closest transiting planet to Earth, which
makes it ideal for follow-up studies. Moreover, it is the nearest rocky planet
confirmed outside our solar system. Its host star is visible to the unaided eye
from dark skies, meaning anyone with a good star map can see this
record-breaking system.
"Most of the known planets are hundreds of light-years
away. This one is practically a next-door neighbor," said astronomer Lars
A. Buchhave of the Harvard-Smithsonian Center for Astrophysics (CfA).
"Its proximity makes HD 219134 ideal for future
studies. The James Webb Space Telescope and future large ground-based
observatories are sure to point at it and examine it in detail," said lead
author Ati Motalebi of the Geneva Observatory.
The newfound world, designated HD 219134b, was discovered
using the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo
in the Canary Islands. The CfA is a major partner with the Geneva Observatory
on the HARPS-North Collaboration, which includes several other European
partners.
HARPS-North detects planets using the radial velocity
method, which allows astronomers to measure a planet's mass. HD 219134b weighs
4.5 times the mass of Earth, making it a super-Earth.
With such a close orbit, researchers realized that there was
good possibility the planet would transit its star. In April of this year they
targeted the system with NASA's Spitzer Space Telescope. At the appropriate
time, the star dimmed slightly as the planet crossed the star's face. Measuring
the depth of the transit gave the planet's size, which is 1.6 times Earth. As a
result, the team can calculate the planet's density, which works out to about 6
g/cm3. This shows that HD 219134b is a rocky world.