Several news stories
like
this one have mentioned the
recent announcement that a planet may have imaged around another
star. If this is true, it would be the first direct picture of a planet
in another solar system. So, is it true?
There are good reasons to be wary. Previous announcements have claimed to
have imaged planets. Some have been shown to be background stars only
appearing close to the "parent" star, others
are still unconfirmed. This potential planet is somewhat exciting because
the astronomers claim to have detected water. In order for water to form,
the object has to be cooler than any known star.
So, how can we be certain that this is a planet orbiting another star? We
could watch to see it orbit the parent star, but that could take hundreds
of years. Or, we can wait a couple of years and look again -- stars all
move relative to one another in the sky. If the purported planet and its
parent star are moving the same direction at the same speed, then they are
certainly travelling together. We must be patient!
This object raises some interesting questions. Assuming it is really a
planet, the planet orbits a brown
dwarf, a "failed" star that has too little mass to make its own light by
nuclear fusion. But most astronomers would agree that a brown dwarf would
not have enough material around it to form planets by "accretion" (Dust
clumps together to form rocks, which clump together to form asteroids,
which clump together to form planets.) So, if this is a planet orbiting a
brown dwarf, it must have formed some other way.
Stars form when giant clouds of gas collapse due to gravity. It is
possible that, when the gas cloud is collapsing, it will split into
several smaller pieces. So it could be that the gas cloud that formed the
brown dwarf split in two, with the larger clump forming the brown dwarf
and the smaller lump the planet. But would you then call the "planet" a
planet? It was made in a different way and likely has a different
structure inside -- Jupiter probably has a center several times the size
of the Earth made of rock, but a "planet" formed by splitting gas clouds
would be almost pure gas, like the sun.
All this may be is a question of defining what a "planet" is. Of course,
if the supposed planet turns out to be a star in the background, then the
whole issue will go away. At least until the next possible planet is
found!