Page last updated at 19:00 GMT, Thursday, 13 November 2008

Exoplanets finally come into view

Three exoplanets (NRC-HIA/ IDPS/ Keck)
Three exoplanets orbiting the same star have been imaged directly

The first pictures of planets outside our Solar System have been taken, two groups report in the journal Science.

Visible and infrared images have been snapped of a planet orbiting a star 25 light-years away.

The planet is believed to be the coolest, lowest-mass object ever seen outside our own solar neighbourhood.

In a separate study, an exoplanetary system, comprising three planets, has been directly imaged, circling a star in the constellation Pegasus.

While several claims have been made to such direct detection before, they have later been proven wrong or await confirmation.

It's a profound and overwhelming experience to lay eyes on a planet never before seen
Paul Kalas, University of California

The search for exoplanets has up to now depended on detecting either the wobble they induce in their parent star or, if their orbits are side-on to telescopes, watching them dim the star's light as they pass in front of it.

Being able to directly detect the light from these planets will allow astronomers to study their composition and atmospheres in detail.

Ring cleaning

The difficulty for astronomers imaging exoplanets is that their parent star's light swamps them - like trying to spot a match next to a floodlight at a distance of a mile.

The light from the star Fomalhaut was blocked to spot the planet

But advances in optics and image processing have allowed astronomers to effectively subtract the bright light from stars, leaving behind light from the planets. That light can either come in the infrared, caused by the planets' heat, or be reflected starlight.

Paul Kalas of the University of California, Berkeley, led an international group that used the Hubble Space Telescope to image the region around a star called Fomalhaut in the constellation Piscis Austrinus.

The star has a massive ring of dust surrounding it that appears to have a cleanly groomed inner edge.

That is in keeping with what is known as accretion theory - that young planets gather up dust and matter as they orbit - and prompted the team to begin looking for the suspected planet in 2005.

The team estimates that the planet, designated Fomalhaut b, is some 18 billion kilometres (11 billion miles) away from its star, about as massive as Jupiter and completes an orbit in about 870 years. It may also have a ring around it.

"I nearly had a heart attack at the end of May when I confirmed that Fomalhaut b orbits its parent star," Dr Kalas said. "It's a profound and overwhelming experience to lay eyes on a planet never before seen."


An artist's concept of the star Fomalhaut and the planet observed by Hubble

Christian Marois of the Herzberg Institute for Astrophysics, Canada, and his team used the Keck and Gemini telescopes in Hawaii to look near a star called HR 8799, which is just visible to the naked eye.

The team studied light in the infrared part of the spectrum, hoping to spot planets that were still hot from their formation.

What they found in 2004, and confirmed again this year, are three planets circling the star.

According to a theoretical model that accounts for the light coming from the planets, they range in size from five to 13 times the mass of Jupiter and are probably only about 60 million years old.

The trio have similarities with our own Solar System. Their orbits are comparable in size to those of the outer planets, and the smaller planets are those closest to the Sun - again suggesting a system that formed through accretion.

Dr Marois points out that the current methods used in the exoplanet hunt are sensitive primarily to Jupiter-sized planets and larger.

"We thus do not have a full picture," he told BBC News. "The detection of the three planets around HR 8799 does not mean that no planets are orbiting at smaller separations. Other gas giant or even rocky planets could reside there."

Comparison - inner four planets in our system not shown; Dwarf planet Pluto included to help see scale (C. Marois/NRC)
A to-scale comparison of the HR 8799 system and our own

The study of the light directly from the planets will yield information about their atmospheres and surfaces that is impossible to collect from planets discovered indirectly.

Further, the current results will also support theories of how planets form from the grand discs of dust and material around stars, and lead to better estimates of how many Earth-like planets are likely to exist.

These latest claims are both based on observations that were well-spaced in time, allowing the researchers to apply a rigorous test for direct detection.

"You see an object next to a star and you might think it's a planet," commented Mark McCaughrean, an astrophysicist at the University of Exeter, UK.

"But you have to watch it for several years and make sure that it moves around the star and with the star as it moves across the sky. Though I've been very sceptical in the past, these ones all seem pretty real to me," he told BBC News

"It's like a London bus - you've been waiting for one for ages and suddenly four come along at once."

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