Astronomers may have discovered 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.
The crowded solar system is described in two papers published on the pre-print server Arxiv.org.
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 Dr. Chris Lintott, from the University of Oxford, co-author on the Planet Hunters paper.
Astronomers may have discovered one of the richest planetary systems yet
“With a transiting system, once you get multiple planets, the odds of them being false positives are very small.”
Dr. Chris 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 neighborhood.
“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 Robert Simpson.
However, the Planet Hunters team carried out simulations showing that the planetary system should be a stable one.
Dr. Chris Lintott 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.
Astronomers say that one in six stars host an Earth-sized planet in a close orbit – suggesting a total of 17 billion such planets in our galaxy.
The result comes from an analysis of planet candidates gathered by NASA’s Kepler space observatory.
The Kepler scientists also announced 461 new planet candidates, bringing the satellites’ total haul to 2,740.
Their findings were announced at the 221st meeting of the American Astronomical Society in California.
Since its launch into orbit in 2009, Kepler has stared at a fixed part of the sky, peering at more than 150,000 stars in its field of view.
It detects the minute dip in light coming from a star if a planet passes in front of it, in what is called a transit.
But it is a tricky measurement to make, with the total light changing just tiny fractions of a percent, and not every dip in light is due to a planet.
So Francois Fressin of the Harvard-Smithsonian Center for Astrophysics – who discovered the first Earth-sized planets set about trying to find out not only which Kepler candidates might not be planets, but also which planets might not have been visible to Kepler.
Artistic illustration of different types of planets in the Milky Way detected by NASA Kepler spacecraft
“We have to correct for two things – first [the Kepler candidate list] is incomplete,” said Francois Fressin.
“We only see the planets that are transiting their host stars, stars that happen to have a planet that is well-aligned for us to see it, and [for each of those] there are dozens that do not.
“The second major correction is in the list of candidates – there are some that are not true planets transiting their host star; they are other astrophysical configurations.”
These might include, for example, binary stars, where one star orbits another, blocking some of the light as the stars transit each other.
“We simulated all the possible configurations we could think of – and we found out that they could only account for 9.5% of Kepler planets, and all the rest are bona fide planets,” Dr. Francois Fressin explained.
The results suggest that 17% of stars host a planet up to 1.25 times the size of the Earth, in close orbits lasting just 85 days or fewer – much like the planet Mercury.
That means our Milky Way galaxy hosts at least 17 billion Earth-sized planets.
Even as Dr. Francois Fressin reported an analysis of the most recent Kepler catalogue, it was increased substantially by results reported by Christopher Burke of the Seti Institute.
Dr. Christopher Burke announced 461 new candidate planets, a substantial fraction of which were Earth-sized or not much larger – planets that have until now been particularly difficult to detect.
“What’s particularly interesting is four new planets – less than twice the size of Earth – that are potentially in the habitable zone, the location around a star where it could potentially have liquid water to sustain life,” said Dr. Christopher Burke.
One of the four, dubbed KOI 172.02, is a mere 1.5 times the size of the Earth and around a star like our own Sun – perhaps as near as the current data allow to finding an “Earth 2.0”.
“It’s very exciting because we’re really starting to pick up the sensitivity to these things in the habitable zone – we’re just really getting to the frontier of potentially life-bearing planets.”
William Borucki, the driving force behind and principal investigator on the Kepler mission, said he was “delighted” by the fresh batch of results.
“The most important thing is the statistics – not to find one Earth but to find 100 Earths. That’s what we’ll be seeing as the years go on with the Kepler mission, because it was designed to find many Earths.”
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