News in Science

Tiny planet rocks astronomy world

Planet find A rocky world smaller than Mercury - the tiniest planet yet detected - has been discovered by an international team of astronomers.

A report in the journal Nature says the planet is one of three discovered orbiting a Sun-like star, 228 light-years away.

"This is a world smaller than any planet in our solar system," says astroseismologist Professor Tim Bedding from the University of Sydney, who was part of the team involved in the discovery.

"It's extremely exciting, until recently scientists had only found exoplanets much larger than Earth. We only began finding Earth-sized planets last year."

According to Bedding, the newly discovered planet named Kepler 37b is about a third the size of Earth, and orbits its host star every 13 days.

Dry lifeless world

"Our best guess is that it would look similar to the Moon or Mercury," says Bedding.

"It would be rocky and might be tidally locked with one side permanently facing its star, so that side would be incredibly hot, around 400° C, while the other would be very cold because it would always be in shadow."

"It's too small to have an atmosphere even if it wasn't so close to its star, so it would probably be a dry, lifeless world."

The planet, is one of three worlds discovered by the team using NASA's Kepler Telescope.

The second planet Kepler 37c, is about 70 per cent the size of Earth and circles the star in 21 days, while the largest of the trio, Kepler 37d, is about twice the size of Earth with a 40-day orbit.

"The host star, Kepler 37a, is older and smaller than the Sun," says Bedding.

"It has about 80 per cent the mass of the Sun, and a surface temperature of about 5,200° C."

"Knowing the star's size, allows us to determine the size of the planet, by the amount of light lost as the planet passes in front of the star."

"We get an indication of the star's size by measuring its oscillation caused by convective gasses moving inside the star, making it vibrate."

Planet hunting

Kepler was launched in 2009 to search for Earth-like worlds orbiting in the habitable zones around Sun-like stars where liquid water could exist on the planet's surface.

It continuously monitors over 150,000 stars looking for changes in the brightness of those stars, indicating a possible orbiting planet passing in front of the star.

"If you get lucky, one of those stars gets a little bit fainter as a planet passes in front of it," says Bedding.

"So far it's only one planet of this size, but theorists have been saying we should expect to find planets that are smaller and smaller."

"And this is the first data point that indicates they might be right."