To form diamonds, there must be the right mix of temperature, pressure and a high concentration of carbon. Imagine if this combination could be found on the inside of a planet.

A carbon planet could possibly have layers of diamond inside that could be miles thick, said Marc Kuchner, a postdoctoral researcher at Princeton University.

Kuchner and some other astrophysicists believe that some extra-solar planets could be carbon planets.

Earth is a silicate planet, meaning it is made up of mostly silicon-oxygen compounds. On a carbon planet, carbon compounds like carbides, diamonds and graphite would be more prevalent, he said.

Carbon planets need a carbon-rich or oxygen-poor environment to form, said Alex Wolszczan, professor of astronomy and astrophysics at Penn State.

"As the universe ages," Wolszczan said, "the chemistry of space changes. It becomes more enriched in heavier elements. These elements spread across space as stars die."

Stars burn hydrogen during the majority of their lives. However, once all of the hydrogen is spent, the core condenses and the burning of helium begins. After the helium-burning stage, the final stage of carbon burning takes place. This is where carbon is released into space, Wolszczan said.

Older areas of the galaxy, which are near the center, are more likely to have carbon planets because the concentration of carbon will be higher, he added.

Wolszczan said with the right amount of pressure, a layer of diamond as thick as Earth's silicate mantle could form.

There are still some discrepancies between theories. Wolszczan said carbon planets could still have iron-nickel cores, while Kuchner said they were more likely to be steel or some steel alloy due to the relative ease that carbon dissolves in metals.

Ways of testing theories about planet formation around distant stars will soon be available. NASA's Terrestrial Planet Finder (TPF) missions, the first of which is set to launch in 2015, will study about 60 stars to gain information about the formation and development of planets outside the solar system, Kuchner said.

Peter Heaney, associate professor of geosciences, said even though this is still speculation, under the right conditions, this kind of diamond formation is possible.

The formation would need a very high carbon-to-oxygen ratio, Heaney said. Otherwise, he said, oxygen will join to create compounds like calcite, also known as calcium carbonate -- the basic component of limestone.

"Diamond and graphite will only form when in an oxygen-poor environment with the right temperature and pressure," he said.

Graphite would form under low pressure, while high pressure would lead to diamonds, Heaney added.

Stable formation of diamonds needs high temperature and high pressure, but "as temperature rises, so must pressure," Heaney said.

He said there is nothing wrong with speculation.

"Speculation helps researchers figure out what they should look for," Kuchner said.

Kuchner was involved with part of the design of the TPF-Coronagraph mission. He designed the band-limited mask that will suppress starlight from the image, allowing for a clearer view of the planets around that star.