As far as we know, extraterrestrial life needs rocky planets to live on. The earliest such planets might have been full of carbon, with early life forms appearing on worlds with layers of diamond under their crusts and coal-black surface rocks.
A recent study by Natalie Mashian and Avi Loeb at the Harvard-Smithsonian Center for Astrophysics looked at the formation of planets around carbon-enhanced metal poor stars (CEMPs). These kinds of stars likely formed in the early universe, just after the first generation of massive stars had burned their nuclear fuel and exploded as supernovae. If there are planets around such stars, it means that life could have appeared in the universe within a couple of hundred million years of the Big Bang, 13.8 billion years ago. Previous studies suggested it might have taken longer; the oldest exoplanet system yet discovered, Kepler 444, surrounds a star that is about 11.2 billion years old.
Elements such as iron and silicon are usually thought of as essential for making planets, because they form dust grains around which bigger bodies can form via gravitational accretion. Even hydrogen-rich gas giants like Jupiter started from such a "seed." However, CEMPs don’t have as many heavy elements like iron as our Sun, only one hundred thousandth as much which is saying something since the Sun is only 0.003 percent iron. So if CEMPs form primarily from clouds of gas and dust of carbon, oxygen, and nitrogen, one question is whether planets like Earth, with solid surfaces, could form.
Mashian and Loeb suggest that planets can in fact accrete in such a nebula, and therefore around CEMPs. Astronomers might find them with some of the latest space telescopes and future instruments, such as the James Webb Space Telescope, as they come on line. "The methods are the same [as for previous exoplanet missions]," Loeb told Smithsonian.com. "You'd look for planets transiting their stars."
In their study Mashian and Loeb model the distances from CEMPs that the planets would form, and how large they are likely to be. Such planets would have little iron and silicon, the elements that make up a large portion of the Earth. Instead they would be richer in carbon. They found the maximum size would tend to be about 4.3 times the radius of the Earth, A carbon planet would, the study says, also allow for a lot of hydrocarbon molecules to form on the surface, provided the temperature isn't too high. And any planet with a mass of less than about 10 times that of Earth would show a lot of carbon monoxide and methane in its atmosphere, the study says.
In a nebula rich in lighter elements, he added that there is also likely to be water, another key component of a biosphere. "Even with low oxygen levels hydrogen tends to combine with it and make water," he said. So a carbon planet might have water present. Loeb said in a statement that since life itself is carbon-based, that bodes well for the appearance of living things.
CEMPs are so poor in heavier elements because they were built from the remains of the first stars to appear in the universe – behemoths with hundreds of times the mass of the sun. A massive star's core is like an onion. The heaviest elements created by nuclear fusion are towards the center – the iron, magnesium and silicon are in the innermost layers, while carbon, oxygen and some remaining helium and hydrogen are in the outer ones. Loeb said much of the material in the inner layers – those heavier elements – will fall back into the black hole that forms after the star becomes a supernova. Meanwhile the lighter elements will be ejected into space to form new stars. Those stars, forming from the gases left over from the first, would be poor in metals like iron, but carbon-rich – the CEMPs.
It is only later, when less massive stars age and explode as supernovae, that the heavier metals can get out. A star below 25 solar masses will collapse into a neutron star or end up as a white dwarf. Unlike black holes, neutron stars and white dwarfs don't have escape velocities faster than light, so the supernova explosion is much more likely to spread the iron from the star's core. That's why stars like the sun have as much iron as they do, and why the Earth has even heavier elements.
Whether such planets have life or not, though, is still an open question. The study itself it is more concerned with getting the planets to form in the first place, which is an essential step for life. "My graduate student [Mashian] is conservative," Loeb quipped. To see signs of life, one needs to see the atmospheres of the planets in question. The target would be the signature of oxygen, which absent some way to replenish it, will disappear from a planet's atmosphere as it reacts with surface rocks. On Earth, the oxygen is made by plants, which take up carbon dioxide. Aliens looking at our own planet's atmosphere would notice something was up.
Seeing those atmospheres – assuming the planets themselves are found – will likely require more powerful telescopes than are available now. "[The James Webb Space Telescope] might marginally do it for the nearest stars," he said. "But CEMPs are ten times further away."