PARIS (AFP) - – Astronomers said on Thursday they had found six
planets orbiting a distant star in the most exciting but also most
challenging find since exploration of other solar systems began 15
years ago.
None of the so-called exoplanets, orbiting the star Kepler-11
2,000 light years away, is remotely comparable to the Earth, but
the detection of them is a major technical feat, the scientists
said.
Five planets are relatively small, ranging in mass from 2.3 to
13.5 times that of Earth, but orbit Kepler-11 at blistering
proximity, encircling it at a distance that is even closer than
Mercury to the Sun.
Their "year" -- the time it takes to encircle the Sun -- is less
than 50 days.
A sixth exoplanet is large and farther out, with an orbital
period of 118 days, and is of a yet-to-be determined mass, although
it is likely to be a "gas giant" like the outer planets of our own
Solar System.
Reporting their find in the British journal Nature, the
astronomers said the star system was mind-boggling because the
planets were so small, so numerous and yet so densely packed.
"We think this is the biggest thing in exoplanets since the
discovery of 51 Pegasi b, the first exoplanet back in 1995," Jack
Lissauer of the NASA-Ames Research Center said in a teleconference
with the media.
Daniel Fabrycky, a post-doctoral researcher at the University of
California at Santa Cruz, described it as "an amazing planetary
system."
"Of the six planets, the most massive are potentially like
Neptune and Uranus, but the three lowest-mass planets are unlike
anything we have in our Solar System," added Jonathan Fortney, the
university's assistant professor of astronomy and astrophysics.
The inner planets seem to have an atmosphere of water or
hydrogen-helium gas. This is another surprise, given the
presumption that a small planet orbiting so close to a star would
have a light atmosphere blasted away by solar radiation.
The extra-solar bodies were found by the US orbiting telescope
Kepler, which already has a fine haul of these so-called exoplanets
to its credit.
Kepler-11's herd of planets orbit in very narrow planes.
Sideways on, as viewed from Earth, "the system is flatter than a
CD," said Lissauer.
The diameter of the planets were calculated using a
highly-sensitive light sensor, a photometer, which detected changes
in the star's brightness when the planets moved in front of it.
This is now a time-honoured approach in exoplanet work. More
than 500 exoplanets have been spotted since 1995, of which around
100 have been detected using this "transit" method.
Where the new work differed is a novel way of calculating the
planet's masses.
Until now, this was done by using ground-based telescopes whose
Doppler spectroscopy measures a "wobble" in starlight caused by the
gravitational tug of the passing planet.
This has been great for measuring very large planets that are,
say, the size of our own Jupiter. But it is not refined enough for
estimating the mass of smaller planets that are little bigger than
Earth.
The team used orbital dynamics to get around this for the
Kepler-11 planets.
They measured tiny variations in the orbital periods caused by
gravitational interaction among the tightly-packed planets
themselves.
Improvements such as this are part of an essential toolkit for
spotting another Earth-like planet which could support life as we
know it.
It would have to be a rocky planet, as opposed to one made out
of gas, and it would have to orbit in the famous Goldilocks Zone,
where the temperature is neither too hot nor too cold, but just
right for water to exist in liquid form.