Searching for a home away from home

Astronomers have discovered a distant star with a complex solar system. They believe the find is as important as the first exoplanet discovered in 1995

ASTRONOMERS who study how planets form around stars have started rethinking their theories following the discovery of a remarkable solar system 2,000 light years away. It breaks the rules and will not be easy to explain, according to scientists studying the find.

“This was not predicted by the models,” according to Jack Lisssauer, a planetary scientist at the US National Aeronautics and Space Administration’s Ames Research Centre in California. “We think this is as big as the discovery of the first exoplanet, 51 Pegasi b, in 1995.”

He was commenting this week during an international press conference to announce the latest findings from the Keplersatellite mission. Launched by Nasa in 2009, Kepleris specifically designed to look for extrasolar planets, planets that orbit distant stars.

Keplerand other observatories have so far found 519 of them, mostly gas giant planets like Jupiter although a few rocky "giant earths" have also been identified.

The hope is we may one day find a match for Earth, a home away from home that might also harbour carbon-based life such as own. Along the way, however, there is a plentiful supply of interesting planets to study.

Most stars only have a single planet around them, but the latest – Kepler-11 (K-11) – amazed astronomers because it has not one but six orbiting planets. Lissauer described K-11 as “the latest and greatest finding to date”.

The K-11 star is about the size of our sun, but at between six and 10 billion years is somewhat older. It is the planets that orbit it however that is causing excitement amongst astronomers.

It is less the number of planets than their orientation around the star. If we drew comparisons with our own solar system, five of the six planets have orbits that are closer to their star than Mercury is to our sun.

Mercury completes an orbit of the sun in about 88 days. The inner five K-11 planets spin around much faster, with short orbits of between just 10 and 47 days, the researchers indicate in a paper published this morning in the journal Nature.

This number of planets clustered and jostling so close together in such a tight space is unprecedented. The discovery was “totally unexpected” because the five planets are so very close. “They are much more closely packed than any other exoplanet system,” says Lissauer. “Something a little different is going on here. It is sending us back to the drawing board.”

It was strange that so much mass had been packed into such a relatively small area and that individual planets had been able to form without either pulling one another apart with gravity or colliding with one another.

The current theories of solar system formation hold that gas and dust left behind by older stars that have been blown apart in a supernova begins to accrete, with more and more material pulled into the centre by gravity. If there is sufficient material and density, the star comes to life, with hydrogen fusion driving the release of energy.

Planets are also thought to accrete in this way with more and more material clumping together. Gas giants form or rocky planets based on the material available as they form.

K-11 seems to fly in the face of this theory however given they are so close together. Their gravity should have interfered with accretion.

“They are low mass planets and have low masses so low density. We are still learning how nature makes medium mass planets,” explains Prof Jonathan Fortney, assistant professor in the department of astronomy and astrophysics at the University of California, Santa Cruz and a senior author of the paper with Lissauer.

He said K-11 provided a “fantastic” laboratory that would allow them to do extensive comparative planetary science because of the interaction between the planets in this system.

The sixth planet is also close to its sun but would be somewhere between the orbits of our Mercury and Venus, says Lissauer. The orbital period for this planet is about 118 days, the researchers said.

The positions of the planets in orbit and sizes relative to Earth and to Jupiter can be seen in the diagram, below right, with the inner five called Kepler-11b, 11c, 11d, 11e and 11f. The outlying planet is Kepler-11g.

The K-11 solar system was also unusual in that the planets orbit in a very tight plane which Lissauer likened to a vinyl record given it was so exceptionally flat.

This flat plane has helped the scientists gather a great deal of data about these planets relatively quickly. Exoplanets are typically discovered when they “transit” their stars, which means they pass in front of them relative to our viewing position.

A transiting planet will block a small amount of the light emitted by the star and scientists can analyse the change in light to estimate the size of the exoplanet.

K-11 has six transiting exoplanets, and the researchers said they had not ruled out the possibility that the star has more planets that do not transit. In this case additional planets could still be identified by watching to see if any hidden planets disturbed the orbits of the six that are visible, the researchers say.

“K-11 is telling us a great deal about planets as individual bodies and as a solar system,” says Lissauer. “How rare is a system like this? It is the only star we have that has six candidates for transiting.” There is a one in 1,000 to one in 10,000 chance of finding another, he adds.

Kepler’s unblinking “eye” stares out into space, constantly monitoring 150,000 stars and watching for a dip in light that means a planet is transiting. Our home away from home is out there and waiting to be discovered.

See kepler.nasa.gov. Twitter: @dickahlstrom