A team of astronomers has found the equivalent to planet Mars in a star
system with three planets 200 light years away.
This is the planet with the lowest mass found so far around any normal
star. That’s because the methods that
allow us to find the masses of planets (how much they weigh) generally work
best for planets that are heavier. Here
a wonderful combination of circumstances allowed the team (including Jason Rowe
of the SETI Institute) to make their record-breaking measurement.
The nameless star system is given the catalog designation Kepler 138,
because the three planets, orbiting a cooler star, were first discovered by the
Kepler spacecraft. Kepler allows
astronomers to find planets when they move across the face of their stars,
causing a tiny eclipse (or “transit.”)
Tremendously accurate instruments aboard Kepler measured the decrease of
light when each planet got in front of the star.
The three planets are each closer to their star than Mercury is to our
Sun. The planet that resembles Mars,
closest of the three, takes only 10.3 days to go around. In other words, a year on that planet is 10
Earth days. Think how often you’d have
to celebrate annual events, like your birthday!
The present team of astronomers (which also includes members from NASA’s
Ames Research Center and Penn State) followed the three planets’ transits over
time and noticed that they did not occur at the same time each orbit, because
the gravity of the other two close planets was tugging on each one. By measuring the size of the tugs, the
astronomers could derive the gravity (mass) of each planet, something that is
otherwise very hard to do.
Now here is the clever part.
When we watch a planet go in front of its star, that allows to measure
how big the planet is (its diameter).
Bigger planets block more light.
So for these three planets, we now had the mass (from the tugs) and the
size (from the transits). Most of the
time, when they find planets around other stars, astronomers only have one OR
the other.
Since we have both in this case, that allows us to calculate the density
of each world. If a planet is dense, it is likely to made mostly of rock, like
Earth is. If a planet is not so dense,
it combines rock with ice or even perhaps melted ice, such as the liquids we
find inside Jupiter and the other partly liquid planets in our solar system.
This is what allowed the team to say with some certainty that the inner
planet in the Kepler 138 system is about the size of Mars and about the same
composition as Mars. The planet is
roughly 10% the mass of Earth and half the size of Earth, just like Mars
is. This is the smallest world for
which we have both size and mass.
Almost 2000 planets are now known around other stars, a remarkable
number, given that the first one was discovered just 20 years ago. What amazes us is the variety of planets out
there. There are huge planets, bigger
than Jupiter, but orbiting very close to their stars. There are planets we are calling
super-Earths, that are intermediate in size between Earth and Neptune. And now we know that there are smaller, solid
worlds like Mars. Some smaller worlds
are really close to their stars, like the one around Kepler 138, but others are
much further out, like Mars is in our solar system. Nature likes diversity in astronomical
settings, much as she likes it for people.
(For a nice "infographic" about the Kepler 138 system compared with our solar system, see:
