As life-forms, humans are carbon-based but our planet is not. Less than half a percent of Earth is carbon. However, we have discovered planets largely formed from the sixth element circling other stars, and a new theory holds that these were suitable places to host life long before Earth was born. 

The planet 55 Cancri e is thought to be largely carbon, possibly making its core a giant diamond. Two years ago, modeling suggested such worlds might be quite common.

At Harvard University, graduate student Natalie Mashian and her supervisor Professor Avi Loeb demonstrated that such planets are likely around carbon-enhanced metal-poor (CEMP) stars. CEMP stars can form very early in the evolution of a galaxy, potentially making these some of the earliest planets.

"This work shows that even stars with a tiny fraction of the carbon in our solar system can host planets," said Mashian in a statement.

The first stars were formed from hydrogen, helium, and small amounts of lithium. No larger atoms were created in the Big Bang. However, as these stars evolved they produced heavier elements, primarily in supernovae explosions. Such explosions distribute these elements into the neighborhood where they can become incorporated into a following generation of stars. However, the first of these “second generation” stars had only small amounts of such elements, carbon included. It is only as time passes and more supernovae go off that stars like the Sun, with higher concentrations of elements such as iron, appear.

The protoplanetary disk around CEMP stars lack the iron and silicon to form planets like Earth, Mashian and Loeb argue, but grains of carbon would clump together to form solid, if low-density, planets.

The pair calculated the distance from the star at which such planets might form during the limited lifetime these disks survive, which for low metal disks is thought to be less than a million years. The higher the metal content of the disk, the further out the planet could form. Noting that CEMP stars typically have masses lower than the Sun, many of these planets could lie in their star's habitable zone.

So might these planets host life? Mashian thinks so. "We have good reason to believe that alien life will be carbon-based, like life on Earth, so this also bodes well for the possibility of life in the early universe," she said.

Identifying such planets from a distance is challenging. However, Mashian and Loeb argue their atmospheres will be rich in carbon monoxide and methane, which we may be able to detect if the planet passes in front of its star. 

The theory of panspermia holds that life came to Earth from outer space. This pushes the question of where and how life might have evolved first back one level. The existence of life on carbon-based planets certainly wouldn't prove the theory, but would expand the possible circumstances in which life could appear while providing plenty of time to reach Earth.