Search for other Earths reveals new planets Four exoplanets with an Earth-like composition have been found by an EU-funded project working to understand more about planets outside our solar system. These particular exoplanets are too hot for life, but the observation method that identified them will now help determine how common inhabitable planets are in our galaxy.
Image of planet Earth in Space

© David A. Aguilar (CfA)

Is Earth unique? Earth-sized planets are common in the galaxy, according to NASA space observations. Now scientists in the ETAEARTH project are determining whether any of these planets could support life as we know it. From eight institutes and universities in Italy, the UK, Switzerland and the US, they are combining space- and Earth-based observations to define the properties of exoplanets – planets outside our solar system.

The project team has identified four planets with a similar composition to that of Earth. All are too hot to host life, but their discovery sheds light on the properties of the stars and the planets that orbit them, which could assist the search for Earth analogues. ETAEARTH’s researchers now plan to extend their method to calculate how common potentially habitable Earth-like planets are throughout our galaxy.

“This is the fundamental question of modern astronomy,” says project coordinator Alessandro Sozzetti of Italy’s Istituto Nazionale di Astrofisica. He explains that the new information will help scientists make the right assumptions about the properties of stars and distant planets. “That means we can focus resources on the most promising candidates.”

Information synergies

ETAEARTH studies stars that are similar or smaller in size than the Sun. The team is looking for solid, rocky planets orbiting at the distance necessary to support an atmosphere and liquid surface water – all basic requirements for complex biology to develop.

To do this, it processes observations from three sources: NASA’s Kepler Space Telescope, the European Space Agency’s Gaia space observatory and the Earth-based Northern High Accuracy Radial velocity Planet Searcher (HARPS-N) spectrograph, built and run by Switzerland, the US, Italy and the UK. “Space- and ground-based observations give the best results when analysed together,” says Sozzetti.

Kepler measures the dip in the light from a star as a planet crosses it, while HARPS-N measures the star’s slight wobble as the planet’s gravity pulls at it. The transit in front of a star reveals a planet’s size, the wobble its mass. Direct measurements of the system’s distance from the Sun using Gaia data improve the accuracy assessments of the central star’s physical properties. When all these elements are combined, scientists can calculate the planet’s density and determine its composition with high accuracy.

Astronomy advances

Kepler 78b was the first Earth analogue identified by ETAEARTH. It orbits the Kepler 78 star, about 400 light years from Earth. “This was the first discovery of a planetary object that has a similar mass, radius and density as Earth,” says Sozzetti.

A year later, ETAEARTH’s scientists discovered rocky planets orbiting the Kepler 10 sun – Kepler 10b – which is small like Earth – and Kepler 10c, which is 2.5 times the size and 17 times the mass of Earth. This huge scale of rocky planet was previously thought not to exist, says Sozzetti. “It provides new information about how planets form and evolve.”

In 2015, the ETAEARTH team discovered HD 219134b, only 20 light years from Earth. It is the nearest rocky planet found transiting a star similar to the Sun and it could have an atmosphere. This method could open the way to detecting other planets with an atmosphere from Earth, says Sozzetti.

The project has already published more than 50 scientific papers, and details of two more star systems are close to publication. There over 100 000 candidate stars in the Kepler field, says Sozzetti. “In our remaining year we will combine Kepler, HARPS-N and Gaia information to infer very precisely what fraction of objects around these are Earth-like, then extrapolate this to the whole galaxy.”

ETAEARTH is making a name for Europe in the planetary sciences, and has established a framework for future cooperation between European and US institutions, he says. “If we make it to the stars, we can thank – in part – ETAEARTH.”

Project details