At a distance of “only” 21 light years from Earth, researchers at the University of Geneva (UNIGE) have uncovered a new planetary system that includes three super-Earths and one giant outer world. One of the so-called super-Earths transits in front of the star and has a density similar to the Earth’s. This makes it the closest transiting planet known, by far.
The planet, known as HD 219134b, moves around its star every three days and is 4.5 times the mass of Earth, and 1.6 times larger, what planet hunters call a super-Earth. Its mean density is close to the density of the Earth, suggesting a possibly similar composition as well.
It’s star, a 5th magnitude K dwarf, slightly colder and less massive than our Sun, is bright enough to follow with a naked eye from dark skies. It can be seen, next to one leg of the W-shape Cassiopeia constellation, all year round in our boreal hemisphere.
First author Ati Motalebi, astronomer at UNIGE, explained:
“When the first HARPS-N radial-velocity measurements indicated the presence of a 3-day planet around HD219134, we immediately asked NASA for Spitzer space telescope time. The idea was to check for a potential transit of the planet in front of the star, a mini eclipse, that would allow us to measure the size of the planet. To do this, we needed to go to space to reach the required precision.”
Astronomers are already planning observations with ground-based high-resolution spectrographs and the future NASA-ESA James Webb Space telescope (JWST) using transmission spectroscopy techniques.
During the transit, the light of the star crosses the atmosphere of the planet on its way to the observer, carrying over the spectral signature of the chemical species present in the atmosphere.
F. Motalebi et al. 2015.
The HARPS-N Rocky Planet Search I. HD219134b: A transiting rocky planet in a multi-planet system at 6.5 pc from the Sun.
A&A, accepted for publication; arXiv: 1507.08532
Illustration: Centre de Données astronomiques de Strasbourg / SIMBAD.