Closest ever transiting super-Earth found 21 light years away

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Astronomers have discovered a planetary system just 21 light years away from us that packs three super-Earths and a gas giant out of which at least one super-Earth is said to be a transiting planet.

Discovered using the HARPS-N, which is installed at the Telescopio Nazionale Galileo on the La Palma island, the exceptional planetary system is located around the star HD219134. The star is a 5th magnitude K dwarf that is slightly colder and less massive than our Sun; however, it is bright enough that it can be spotted with a naked eye from dark skies, next to one leg of the W-shape Cassiopeia constellation, all year round in our boreal hemisphere.

Astronomers from the University of Geneva (UNIGE) and members of the NCCR PlanetS say that the cortege of planets around HD219134 is composed of three mostly rocky super-Earths and an outer giant planet, a configuration reminiscent of our own Solar System.

Ati Motalebi, astronomer at UNIGE and first author of the paper said that as soon as the first HARPS-N radial-velocity measurements were out and indicated the presence of a 3-day planet around HD219134, they requested NASA for Spitzer space telescope time.

Astronomers wanted to check for presence of a potential transit of the planet in front of the star so as to measure the size of the planet.

Using the Spitzer telescope astronomers found that HD219134b does indeed transit the star and it is by far the closest transiting planet known, and likely to remain one of the closest ever.

Calculations reveal that the planet HD219134b is a 4.5 times more massive than the 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.

However, that was not it. Astronomers also found three additional longer-period planets in the system from the HARPS-N radial velocities. In the inner regions, a planet weighing 2.7 times the Earth orbits HD219134 in 6.8 days, and a planet of 8.7 times the mass of the Earth resides on a 46.8-day orbit.

The team speculates that if these 2 planets would be in a coplanar configuration with their 3rd inner sister, as often observed for compact systems, the whole family might be transiting.

They have already arranged for future observations to capture the potential transits. The team is waiting for the European Space Agency’s (ESA) CHEOPS satellite that will provide the perfect tool for such observations.

The story does not stop here, yet. The system includes as well a giant planet (of small-Saturn type) at 2.1 astronomical units, orbiting the star in a bit more than 3 years.

This system, reminiscent of our own Solar System with the inner “small” planets and the outer gaseous one, will without doubt encounter a growing interest from the astronomical community. Indeed, the proximity and brightness of the star makes the system the most favourable one for an in-depth characterisation of the planet physical properties.

For atmospheric studies, 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.

They even dream about direct imaging of the outer planet in the system with the new generation of giant telescopes on the ground, the Extremely Large Telescopes, planned for the next decade.