Astronomers may have identified one of the richest planetary systems yet.
The discovery of a seventh planet around the dwarf star KIC
11442793 could be a record, according to two separate teams of
researchers.
The system bears some similarities to our own, but all seven
planets orbit much closer to their host star, which lies some 2,500
light-years from Earth.
One of the identifications was made by volunteers using the Planet Hunters website.
The site was set up to allow volunteers to sift through the public data
from Nasa's Kepler space telescope - which was launched to search for
so-called exoplanets - worlds orbiting distant stars.
Kepler uses the transit method to discover new planets, which
entails looking for the dip in light as an alien world passes in front
of its host star. But there is simply too much data for mission
scientists to examine every light curve, so they developed computer
programmes to search for the signature of a planetary transit.
"This is the first seven-planet system from Kepler, using a
transiting search. We think [the identification] is very secure," said
Chris Lintott, from the University of Oxford, co-author on the Planet
Hunters paper.
"With a transiting system, once you get multiple planets, the odds of them being false positives are very small."
Exoplanets
Dr Lintott's team has submitted their research
to the Astronomical Journal for peer review. Another team of
astronomers from several European countries has submitted a separate paper outlining their independent discovery of the seventh planet to the Astrophysical Journal.
The new planet is the fifth furthest from its parent star, orbiting with a period of nearly 125 days.
With a radius of 2.8 times that of the Earth, it fits into a
family that now includes two roughly Earth sized worlds, three
"super-Earths" and two larger bodies.
"It actually looks like our Solar System in one sense, with
all the small planets on the inside and the big planets on the outside.
And that's not necessarily what we always see," said co-author Robert
Simpson, also from Oxford University.
While there might be resemblances to our Solar System, all
seven planets are closer to their host star. In fact they would all fit
within the Earth's distance from the Sun, making this a very crowded
neighbourhood.
"This is one of the reasons they are easy to see, because the
closer they are to their sun, the more frequently they go around it,"
said Dr Simpson.
However, the Planet Hunters team carried out simulations showing that the planetary system should be a stable one.
Dr Lintott, who co-presents the BBC's Sky at Night programme
and helped found Planet Hunters, added: "Everything we know about this
system tells us [the seventh planet] should have been found using the
automatic detection routines. But it wasn't."
"A seven-planet system is very complicated so you get a sense
of why the automatic routine might have missed out - it gets confused
by the presence of the other transits.
"Looking for these transits seems like a task that's
perfectly designed for computers. But we keep finding, in these niche
cases, in these odd cases, in these complicated cases that humans can
beat the computers."
Another star, HD 10180, has been claimed to have either
seven, or nine planetary signals. A distant sun called GJ 887C may also
have a family of seven planets.
Commenting on the paper by the Planet Hunters team, Andrew
Collier Cameron, professor of astronomy at the University of St Andrews,
said: "It's intriguing that a system as well-studied as KOI-351 can
still harbour hidden surprises that can only be winkled out by human
eyes."
He added: "This is a perennial problem in transit hunting, of
which we are only too acutely aware in our own ground-based searches.
"The best transit signal-detection algorithms developed to
date still come a very poor second to the human visual system when it
comes to pattern recognition. Still, we have to rely on machines,
because of the sheer volume of data produced by enterprises like transit
searches."
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