Exoplanets finally come into view

source: BBC

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 Enlarge Image

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."

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.

Trio of 'super-Earths' discovered

The planets are rocky worlds between two and 10 times the size of Earth

Astronomers have identified a trio of so-called "super-Earths" - rocky planets between two and 10 times the mass of Earth.

The three new planets were detected using the Harps instrument at the La Silla Observatory in central Chile.

The star they circle is slightly smaller than our Sun, and is located 42 light-years away near the southern Doradus and Pictor constellations.

The discoveries were announced at an astronomy conference in Nantes, France.

When a planet orbits its star, it exerts a gravitational pull which causes the parent star to "wobble" around its centre of mass.

The High Accuracy Radial velocity Planet Searcher (Harps) spectrograph was able to measure this wobble to a very high precision over a period of five years.

This was vital because the perturbations caused by the planets were tiny.

"The mass of the smallest planet is one hundred thousand times smaller than that of the star," said co-author Francois Bouchy, from the Astrophysics Institute of Paris, France.

Chances are

The new worlds, which circle the star HD 40307, are 4.2, 6.7 and 9.4 times the size of Earth. They are named super-Earths because they are more massive than the Earth but less massive than Uranus and Neptune (which are about 15 Earth masses).

Using Harps data, the astronomers also counted a total of 45 candidate planets with a mass below 30 Earth masses.

This implies that one solar-like star out of three harbours such planets.

Astronomer Michel Mayor from the Geneva Observatory in Switzerland commented: "Does every single star harbour planets and, if yes, how many?

"We may not yet know the answer but we are making huge progress towards it."

Since the discovery in 1995 of a planet around the star 51 Pegasi by Michel Mayor and his colleague Didier Queloz, more than 270 exoplanets have been found - mostly around Sun-like stars.

The majority of these planets are gas giants, a bit like Jupiter or Saturn in our own Solar System. Current data shows that about one in 14 stars harbours this kind of planet.

The Harps instrument is attached to the La Silla 3.6m telescope in Chile. The facility is run by the European Southern Observatory (Eso) organisation.

Solar System's 'look-alike' found

By Paul Rincon Science reporter, BBC News, Belfast

Almost 300 planets have now been found outside our Solar System

Astronomers have discovered a planetary system orbiting a distant star which looks much like our own.

They found

two planets that were close matches for Jupiter and Saturn orbiting a star about half the size of our Sun.

Martin Dominik, from St Andrews University in the UK, said the finding suggested systems like our own could be much more common than we thought.

And he told a major meeting that astronomers were on the brink of finding many more of them.

The St Andrews researcher said

this planetary system, and others like it, could host terrestrial planets like Earth.

It was just a matter of time before such worlds were detected, he explained.

Dr Dominik told BBC News: "We found a system with two planets that take the roles of Jupiter and Saturn in our Solar System. These two planets have a similar mass ratio and similar orbital radius and a similar orbital period.

"It looks like this may have formed in a similar way to our Solar System. And if this is the case, it looks like [our] Solar System cannot be unique in the Universe. There should be other similar systems out there which could host terrestrial planets."

Dr Dominik presented his work at the Royal Astronomical Society's National Astronomy Meeting in Belfast.

Ultimate goal

The newfound planetary system, which orbits the star OGLE-2006-BLG-109L, is more compact than our own and is about five thousand light-years away. (Astrophysics - Science journal - Feb 2008 - Full Article pdf)

Although nearly 300 extrasolar planets have been identified, astronomers have consistently failed to find planetary systems which resemble our own. Dr Dominik said only 10% of systems discovered so far are known to host more than one planet.

But he explained that all the

techniques currently used to find exoplanets were strongly biased towards detecting gas giant planets orbiting at short distances from their parent stars.

The OGLE (Optical Gravitational Lensing Experiments) planets were found using a technique called Gravitational Microlensing, in which light from the faraway planets is bent and magnified by the gravity of a foreground object, in this case a another star.

"It's a kind of scaled-down version of our Solar System. The star the planets are orbiting is half as massive as the Sun and they orbit half as distant to their host star as Jupiter and Saturn orbit around the Sun," said Dr Dominik.

He said that

the ultimate goal for exoplanet researchers was to find habitable Earth-like and Mars-like planets. This aim was achievable, he said, because technology was improving all the time.

"I think it will happen quite soon," he said, adding: "Micro-lensing can already go below Earth mass and it has detected more massive planets in the habitable zone. So in the next few years, we will see something really exciting."

Dr Dominik said there was competition between teams of astronomers using micro-lensing and those who favoured the transit technique, which seeks to detect new planets when, from our point of view, they pass directly in front of the parent star they are orbiting. The planet blocks a tiny fraction of the star's light, causing the star to periodically dim.

But he added that there was little chance to detect Earth-like worlds in OGLE-2006-BLG-109L because the system was too distant for current techniques to resolve planets the size of our own.