By Dr David Whitehouse
BBC News Online science editor
US astronomers say they have found two more Neptune-sized planets orbiting stars beyond our Solar System.
It is the fourth planet found around 55 Cancri
They say their discovery of the smallest worlds yet seen circling other stars is a breakthrough in the search for other Earths and for life in space.
They are only about 15 times more massive than the Earth. Previously known worlds were Jupiter-class, some 318 times more massive than the Earth.
One of the new planets is in the first four-planet system ever discovered.
Down to Earth
"We are poised to find true Earth-sized worlds," says planet-hunter Dr Geoff Marcy, of the University of California, Berkeley.
Using telescopes in Hawaii, California and Texas, the astronomers have found the first Neptune-size planets outside our Solar System.
One of them could be only 14 times the mass of the Earth, which may be small enough to have a solid surface and possibly temperatures conducive to life.
"As a result of these observations, we believe that there are 20 billion planetary systems in our Milky Way galaxy " Dr Marcy added.
"It sounds a bit strange, but we're not thinking in terms of Jupiter masses or Saturn masses anymore, but Earth masses."
The new planets are only about 20 times the mass of Earth
They could be rocky, like Earth, or gaseous, like Jupiter
He continued: "All so-called exoplanets found so far are almost certainly gas giants, but these new ones are a puzzle - they could be gaseous like Jupiter, but they also could have a rock-ice core and a thick envelope of hydrogen and helium gas, like Neptune, or they could be a combination of rock and ice, like Mercury."
Dr Marcy and planet-hunting colleague Dr Paul Butler, of the Carnegie Institution of Washington, along with Dr Barbara McArthur, of the University of Texas, leader of the second team, announced their discoveries at a US space agency (Nasa) press conference.
Two papers detailing the find were submitted to the Astrophysical Journal in July and accepted for publication later this year.
The detection of a third Neptune-sized extrasolar planet, not yet peer-reviewed in a journal, was announced by European astronomers last week.
All the planets were discovered using the familiar "wobble technique": a planet's gravitational tug on its parent star will produce changes in the star's velocity. This can be picked up as a Doppler shift in the light emitted by the star.
The nature of that signal can reveal details such as the mass and orbital period of the planet.
The US researchers point out that their new planets, although approaching the Earth in size, are far from Earth-like.
Both whip around their stars in a few days and are so close as to be roasting on the side facing the star.
Astronomers are edging down in size for the exoplanets
The smaller of the two planets, with a minimum mass of 14 Earth masses, was discovered by Dr McArthur and her team around the star 55 Cancri.
It is the fourth planet found around 55 Cancri, a yellow G-class star not unlike the Sun and only 41 light-years from Earth in the constellation Cancer. "There could be other planets in this system as well," Dr McArthur said.
The larger planet, with a minimum size 21 times the mass of the Earth, orbits a red M star, Gliese 436, which is about 50 times dimmer than our Sun and located 33 light-years away in the constellation Leo.
It orbits very close to its star. If the planet has little atmosphere to spread the heat around, it is likely to have temperatures of 377C on the side facing the star, where it would be perpetual noon, and a frigid few tens of degrees above absolute zero where it is perpetual midnight.
However, temperatures on the twilit border could be more comfortable, at least from an Earthling's perspective, Dr Marcy said.
"The front is hot and the backside is probably cold, but the region in between could have moderate temperatures between 0 and 100 degrees Celsius," he added.
The near-simultaneous discovery of these smallest-yet planets indicates they could be common, Dr Marcy believes.
"If you look at the 135 or so extrasolar planets found so far, it's clear that nature makes more of the smaller planets than the larger ones," he explained.
"We've found more Saturn-size planets than Jupiter-size planets, and now it appears there are more Neptune-size planets than Saturn-size. That means there's an even better chance of finding Earths, and maybe more of them than all the other planets we've found so far.
"This is a real breakthrough," Dr Marcy added. "We should be able to easily detect planets only 10 times the Earth's mass. I expect we'll find dozens of planets between 10 and 20 Earth masses in the next few years."