In the video below they discuss how exoplanets could give clues to what earth may have look liked in past:

Recently, another team of researchers suggested that GJ 436b might possess a helium-rich sky depleted of hydrogen. “However, in order to be really hydrogen-poor and helium-rich, the atmosphere of GJ 436b should have represented a very small fraction of the planet[‘s] initial mass, around one-thousandth,” Ehrenreich said. “In such a case, the whole atmosphere would have been gone today, which as we measure is not the case.”

Who really knows for sure?  How about a theory that this could be what a planet can be like in the future if it was pulled in a weird orbit?  What if it was much larger and then spit off so much debris and gases it turned into a more comet like phenomenon?

We hope you like the video!

thanks to livescience for the great info