Radioactive metal might help explain why Earth and its sibling worlds are not ocean planets hostile to life as we know it, a new study finds.
Earth is the only world known to both have life and possess water covering most of it. Since there is life virtually wherever there is water on Earth, the hunt for potentially habitable planets is largely focused on worlds that might have water on their surfaces.
Although nearly three-quarters of Earth's surface is covered in oceans, seas, rivers and lakes, the other planets in the inner solar system — Mercury, Venus and Mars — are relatively poor in water. However, recent findings suggest that most rocky exoplanets may actually be ocean planets, suggesting the solar system is statistically unusual. [10 Exoplanets That Could Host Alien Life]
Possessing vast amounts of water might actually prove hostile to life as we currently know it. Ocean planets are covered by deep global oceans and dense layers of ice on their ocean floors. "Currently we have only an extremely limited understanding whether such worlds can develop life as we know it," senior study author Tim Lichtenberg, a planetary physicist at the University of Oxford in England, told Space.com.
To see how much water exoplanets might generally possess, researchers examined the heat generated by radioactive metals within planetesimals, the building blocks of planets. Specifically, they looked at the radioactive isotope aluminum-26, whose atoms each have one less neutron in their nuclei than atoms of regular aluminum.
Previous research suggested that when the early sun formed about 4.6 billion years ago, a nearby supernova seeded the newborn solar system with aluminum-26. The heat from that radioactive metal helped power the evolution of the interior of planetesimals in the young solar system, dehydrating them.
The scientists ran 540,000 computer simulations, each one involving a moon-size protoplanet that grew from the accretion of gas and of planetesimals 0.6 to 60 miles (1 to 100 kilometers) in size. Across the simulations, the researchers varied the kind of star these protoplanets orbited, the starting distance of these worlds from their stars and the amounts of ice and aluminum-26 they accumulated.
All in all, the scientists discovered planetary systems with rocky planets divided into two main groups — ones poor in aluminum-26 that mostly formed ocean planets, and ones rich in aluminum-26 like the solar system, which formed small, water-depleted worlds. The researchers found that rocky exoplanets poor in aluminum-26 can prove so rich in water, they could be about 10 percent larger in diameter than rocky exoplanets rich in aluminum-26, because water is less dense than rock.
Future research can investigate how aluminum-26 may influence the evolution of giant planets such as Jupiter. In addition, "I am interested in the main factors that control whether a growing protoplanet can emerge as a potentially habitable world that is conducive to life as we know," Lichtenberg said. "We will need to learn so much more in order to come closer to an understanding of how many planets similar to Earth are out there in the galaxy."
The scientists detailed their findings online Feb. 11 in the journal Nature Astronomy.
This story was updated at 3:30 p.m. EST on Feb. 12 to include comments from the science team.