Exoplanet tally soars above 1,000
22 October 2013
Last updated at 13:30 GMT By Melissa Hogenboom Science reporter,
BBC News
Scientists are looking for more rocky worlds that
could possibly retain liquid water on their surface The number of observed exoplanets - worlds circling
distant stars - has passed 1,000.
Of these, 12 could be habitable - orbiting at a distance where it is neither
"too hot" nor "too cold" for water to be liquid on the surface.
The planets are given away by tiny dips in light as they pass in front of
their stars or through gravitational "tugs" on the star from an orbiting
world.
These new worlds are listed in the Extrasolar Planets
Encyclopaedia.
The tally now stands at 1,010 new exoplanets.
Abel Mendez of the Planetary Habitability Laboratory at the University of
Puerto Rico, said that although the number has rapidly increased in recent
years, due to a lack of funding this figure is much lower than it could be.
"We have more techniques and proven technology to detect more exoplanets, but
the limit has been telescopes, especially space telescopes.
"If we had more funding there would be more telescopes and that count would
be much larger by now."
The Kepler space telescope, which spotted many of these worlds in recent
years, broke down earlier this year. Scientists still have to trawl through more
than 3,500 other candidates from this mission so the number could rapidly
increase.
In January 2013, astronomers used Kepler's data to estimate that there could
be at least 17 billion
Earth-sized exoplanets in the Milky Way galaxy. They said that one in six
stars could host an Earth-sized planet in close orbit.
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Exoplanets
- An exoplanet exists outside our Solar System
- Many of those found so far are large planets believed to resemble Jupiter or
Neptune
- The first exoplanet was discovered in 1992, orbiting a pulsar
- A few years later, the planet 51 Pegasi B was found
orbiting a star similar to the Sun
- Hundreds of extrasolar planets have been confirmed since, and thousands more
"candidates" await confirmation
The number of confirmed planets frequently increases because as scientists
analyse the data they are able publish their results online immediately. But as
the finds are not yet peer reviewed, the total figure remains subject to
change.
"Each night we get a list of astronomy papers where there might be an
exoplanet announcement. When we get that we have to review it," explained Prof
Mendez.
This exoplanet catalogue is organised by Jean Schneider, an astronomer at the
Paris Observatory. For the past 18 years he has catalogued new exoplanets on the
Extrasolar Planets Encyclopaedia.
Fuzzy
planets
Others in the field, like Prof Mendez, are then able to review and comment on
the findings, which makes it "more dynamic" he explained.
"That's why the other catalogues just lag behind. The review is reliable as
it's exactly the same as what the journals do." Prof Mendez told BBC News.
Nasa will only accept those announced in an academic journal, so it updates
its list far less frequently. Their tally currently stands at 919 confirmed
worlds.
Jean Schneider said it was also important to note that there was "no
consensus for the definition of a planet" and that past experience had shown
that a few objects declared as planets were actually artefacts or low-mass
stars.
"Some objects, like some Kepler planets, are declared 'confirmed planets' but
have not been published in [referenced] articles. It does not mean that they
will not be published later on, but it introduces another fuzziness in the
tally," he added.
Even if there was a generally adopted definition, Mr Schneider said that for
some objects "there is a large uncertainty on parameters, so that the planetary
nature of the object is uncertain".
For Prof Mendez reaching 1,000 marks an important milestone in the quest to
understand the history of the evolution of the cosmos.
A planet next door
"I don't just want to know where the exoplanets are, I want to understand the
stars, because they are the hosts for the planets. I want to understand the
whole galaxy and the distribution of the stars because everything is connected,"
he explained.
For him, the most exciting discoveries are Earth-like planets which could be
habitable.
"We want to know how unique our planet is, that's a big question and we are
now closer than ever," he added.
For Mr Schneider the most interesting is the candidate
exoplanet around Alpha Centauri, as it is circling a star only four
light-years away.
This planet likely has the same mass as Earth but is outside the "habitable
zone" as it circles its star far closer than Mercury orbits our Sun.
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