In the on-going search for habitable exoplanets, astronomers have been searching for evidence of how planets orbiting distant stars were created. Although planet Earth’s “test kitchen” has provided a detailed recipe, it hasn’t been clear whether other planetary systems followed the same formula.
Now, researchers are reporting evidence that the formula for Earth also applies to terrestrial exoplanets orbiting distant stars.
“Our solar system is not as unique as we might have thought,” says lead author Courtney Dressing of the Harvard-Smithsonian Center for Astrophysics (CfA). “It looks like rocky exoplanets use the same basic ingredients.” Dressing presented the research on January 5, 2015, at a meeting of the American Astronomical Society.
The key to the discovery was the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands. HARPS-N (High Accuracy Radial velocity Planet Searcher in North hemisphere) is an echelle spectrograph covering the wavelength range between 383 to 693 nm, with a spectral resolution R=115000. This instrument allows the measurement of radial velocities with the highest accuracy currently available in the north hemisphere and is designed to avoid spectral drift due to temperature and air pressure variations thanks to a very accurate control of pressure and temperature. The main scientific rationale of HARPS-N is the characterization and discovery of terrestrial planets by combining transits and Doppler measurements.
Designed to accurately measure the masses of small, Earth-sized worlds, the instrument’s measurements are crucial to determine densities and, therefore, compositions.
“Our strategy for using HARPS-North over the past year has been to focus on planets less than two times the diameter of Earth and to study a few planets really well,” explains Harvard astronomer David Charbonneau (CfA), who currently heads up the HARPS-North Science Team.
Most recently, the team targeted Kepler-93b, a planet 1.5 times the size of Earth in a tight, 4.7-day orbit around its star. The mass and composition of this world were uncertain. The researchers report that HARPS-North nailed the mass at 4.02 times Earth, meaning that the planet has a rocky composition. They then compared all 10 known exoplanets with a diameter less than 2.7 times Earth’s that had accurately measured masses, and they found that the five planets with diameters smaller than 1.6 times Earth showed a tight relationship between mass and size. Moreover, Dressing and her colleagues found that Venus and Earth fit onto the same line, suggesting that all these worlds have similar rock-iron compositions.
As for the larger and more massive exoplanets, their densities proved to be significantly lower, meaning that they include a large fraction of water or other volatiles, hydrogen and/or helium. They also showed more diverse compositions rather than fitting into a single group like the smaller terrestrial worlds.
The team also noted that not all planets less than six times the mass of Earth are rocky. Some low-mass worlds with very low densities are known (such as the planets in the Kepler-11 system). But for typical close-in small planets, the chances are high that they share an Earth-like composition.
“To find a truly Earth-like world, we should focus on planets less than 1.6 times the size of Earth, because those are the rocky worlds,” recommends Dressing.
The paper presenting this work has been accepted for publication in The Astrophysical Journal.
The Harvard-Smithsonian Center for Astrophysics (CfA) is a joint collaboration between the Smithsonian Astrophysical Observatory and the Harvard College Observatory. CfA scientists, organized into six research divisions, study the origin, evolution and ultimate fate of the universe.
