Almost 200 planets have been discovered orbiting other stars in recent years. All of them are giant planets and most were found using indirect measurements such as how their gravity made their parent stars wobble. Finding smaller, Earth-sized planets is impossible using current methods. Webster Cash, director of the Centre for Astrophysics and Space Astronomy at the University of Colorado in Boulder, designed a way to get around the problem: a pair of spacecraft, a starshade (the astronomical equivalent of sunglasses) and a telescope that together work as a giant pinhole camera called the New Worlds Observer (NWO). The shade would be shaped like the petals of a sunflower, a shape that Professor Cash calculated would be most effective for blocking the light of distant stars. Any planets around the blocked star, even if they were as small as the Earth, would show up as bright specks of light.

Prof Cash's design is published in Nature today. Writing in the journal, he said that science fiction writers had speculated on the existence of Earth-like planets around nearby stars for more than a century. "If they actually exist, use of [NWO] could find them within the next decade."

The starshade would sit 15,000 miles from the telescope and could be moved into position when the NWO is pointed at a particular star. The setup would orbit at around a million miles from Earth and would allow scientists to spot Earth-sized planets around stars more than 30 light years away. The NWO could resolve details as small as 60 miles across on other planets, giving scientists their first views of the clouds, oceans and continents on planets far from our solar system. "We are hopeful that starshades will enable an entirely new field of astronomy to blossom," Prof Cash said. "Once the planets are visible, free of interfering starlight, the full power of astronomical instrumentation can be unleashed upon them."

He added that the NWO could be built within six years at a cost of $400m (around £220m) and has already put it forward as an add-on to Nasa's proposed successor for the Hubble space telescope, the James Webb space telescope, due for launch in 2013. Around 10,000 stars would be in range of the NWO and each may have up to 10 planets. "If Earth-like planets are common in the universe, then we will find dozens. If they are rare, then we'll be disappointed, but at least we need to know that. Would it help people to take care of the Earth seriously if we find there is actually nowhere else to go?"

Cassini passed between the F and G rings during its insertion into orbit in late June 2004. The G ring's arc is the same feature identified in images taken in May 2005.

"We have seen the arc a handful of times over the past year," said Cassini team member Matt Hedman of Cornell University in Ithaca, N.Y. "It always appears to be a few times brighter than the rest of the G ring and very tightly confined to a narrow strip along the inside edge of the 'normal' G ring."

Imaging team members now think this feature is long-lived and may be held together by resonant interactions with the moon Mimas of the type that corral the famed ring arcs around Neptune.

"We've known since the days of Voyager that we had Jovian-type and Uranian-type rings within the rings of Saturn," said Cassini imaging team leader Carolyn Porco in Boulder, Colo. "Now it appears that Saturn may be home to Neptunian-type rings as well. Saturn's rings have it all."

Porco was the first to work out the dynamics of the Neptunian arcs from Voyager observations.

The scientists do not know exactly how the bright arc formed. One possibility is collisions between small, possibly meter-sized icy bodies orbiting within the G ring set loose a cloud of fine particles that eventually came under the influence of the moon Mimas.

The new observation, however, suggests the remainder of the G ring itself might be derived from particles leaking away from this arc and drifting outwards. Future Cassini imaging observations are being planned to take a closer look at the G ring arc.

Results from Cassini's previous encounters with Enceladus indicated its south polar geysers as the primary source of the E ring particles. Now, images of the E ring with finer resolution than has ever been obtained before show telling details that appear to confirm this relationship.

The new images, taken when Cassini was in the ring plane and consequently showing an edge-on view, reveal a double-banded appearance to the ring, created because the ring is somewhat fainter close to the ring plane than it is 500-1,000 kilometers (300-600 miles) above and below.

This appearance can result if the particles comprising the ring circle Saturn on inclined orbits with a very restricted range of inclinations. A similar effect is seen in the Jupiter's gossamer ring and in the bands of dust found within the main asteroid belt.

Scientists think this special condition might arise for two reasons: The particles escaping from Enceladus and settling into Saturn orbit could begin their journey within a restricted range of velocities and therefore inclinations.

Alternatively, the particles could begin with a large range of inclinations, but those orbiting very close to the ring plane get gravitationally scattered and removed from that region. Future studies of the E ring, including observations and dynamical models, should decide this issue.

"We'll want images from a few other vantage points to be sure of the structure, and then we can test several models to see why these ring particles end up in such a distinct configuration," said Cassini imaging team member Joseph Burns, also of Cornell.

"It is clear that seawater chemistry will change in coming decades and centuries in ways that will dramatically alter marine life," said Joan Kleypas, the report's lead author. "But we are only beginning to understand the complex interactions between large-scale chemistry changes and marine ecology. It is vital to develop research strategies to better understand the long-term vulnerabilities of sensitive marine organisms to these changes."

The report warns that oceans worldwide absorbed approximately 118 billion metric tons of carbon between 1800 and 1994, making them less alkaline and more acidic.

That increased acidity lowers the concentration of carbonate ion, a building block of the calcium carbonate that many marine organisms use to grow their skeletons and to create coral reef structures.

"This is leading to the most dramatic changes in marine chemistry in at least the past 650,000 years," said oceanographer Richard Feely.