Earth-Like Planets Are More Common Than Thought

New research suggests that Earth-like planets can form in a much wider variety of star systems than previously thought. It had been thought rocky planets could only form around stars with a large amount of heavier elements, therefore excluding newer star-producing regions in the universe.

The new research means that Earth-like planets could have formed very early in the universe’s history.

“This work suggests that terrestrial worlds could form at almost any time in our galaxy’s history,” said Smithsonian astronomer David Latham (Harvard-Smithsonian Center for Astrophysics). “You don’t need many earlier generations of stars.”

In astronomy, elements heavier than hydrogen and helium are called “metals.” Stars have their “metallicity” measured and categorized, using the sun as a benchmark. Stars that have more heavy elements are considered metal-rich, ones with less are metal-poor.

For the research, more than 150 stars known to have planets were analyzed. The researchers measured the stars “metallicities” and correlated that with the size of the associated planets. Larger planets tended to orbit stars with as much or more metallicity than the Earth, while smaller planets were found around metal-rich and metal-poor stars.

“Giant planets prefer metal-rich stars. Little ones don’t,” explained Latham.

The research found that Earth-like planets formed at a wide range of metallicities, including around stars with only one-fourth the metal content of our sun.

“Their discovery supports the ‘core accretion’ model of planet formation. In this model, primordial dust accumulates into mile-sized planetesimals that then coalesce into full-fledged planets. The largest, weighing 10 times Earth, can then gather surrounding hydrogen and become a gas giant.”

The core of a gas giant must form quickly since the hydrogen in the proto-planetary disk disappears rapidly. It disappears in just a few million years, being swept away by stellar winds.

“Higher metallicities might support the formation of large cores, explaining why we’re more likely to find a gas giant orbiting a metal-rich star.”

“This result fits with the core accretion model of planet formation in a natural way,” said Latham.

Source: Harvard-Smithsonian Center For Astrophysics

Image Credits: University of Copenhagen/Lars Buchhave, : NASA Goddard Space Flight Center Image by Reto Stöckli (land surface, shallow water, clouds). Enhancements by Robert Simmon (ocean color, compositing, 3D globes, animation). Data and technical support: MODIS Land Group; MODIS Science Data Support Team; MODIS Atmosphere Group; MODIS Ocean Group Additional data: USGS EROS Data Center (topography); USGS Terrestrial Remote Sensing Flagstaff Field Center (Antarctica); Defense Meteorological Satellite Program (city lights))