Time Space Science

An extrasolar planet under three suns has been discovered in the constellation Cygnus by a planetary scientist at the California Institute of Technology using the 10-meter Keck I telescope in Hawaii. The planet is slightly larger than Jupiter and, given that it has to contend with the gravitational pull of three bodies, promises to seriously challenge our current understanding of how planets are formed.

In the July 14 issue of Nature, Maciej Konacki, a senior postdoctoral scholar in planetary science at Caltech, reports on the discovery of the Jupiter-class planet orbiting the main star of the close-triple-star system known as HD 188753. The three stars are about 149 light-years from Earth and are about as close to one another as the distance between the sun and Saturn.

In other words, a viewer there would see three bright suns in the sky. In fact, the sun that the planet orbits would be a very large object in the sky indeed, given that the planet's "year" is only three and a half days long. And it would be yellow, because the main star of HD 188753 is very similar to our own sun. The larger of the other two suns would be orange, and the smaller red.

Konacki refers to the new type of planet as "Tatooine planets," because of the similarity to Luke Skywalker's view of his home planet's sky in the first Star Wars movie.

"The environment in which this planet exists is quite spectacular," says Konacki. "With three suns, the sky view must be out of this world-literally and figuratively."

However, Konacki adds that the fact that a planet can even exist in a multiple-star system is amazing in itself. Binary and multiple stars are quite common in the solar neigborhood, and in fact outnumber single stars by some 20 percent.

Researchers have found most of the extrasolar planets discovered so far by using a precision velocity technique that is easier to employ on studies of single stars. Experts generally avoided close-binary and close-multiple stars because the existing planet detection techniques fail for such complicated systems, and also because theories of solar-system formation suggested that planets were very unlikely to form in such environments.

Konacki's breakthrough was made possible by his development of a novel method that allows him to precisely measure velocities of all members of close-binary and close-multiple-star systems. He used the technique for a search for extrasolar planets in such systems with the Keck I telescope. The planet in the HD 188753 system is the first one from this survey.

"If we believe that the same basic processes lead to the formation of planets around single stars and components of multiple stellar systems, then such processes should be equally feasible, regardless of the presence of stellar companions," Konacki says. "Planets from complicated stellar systems will put our theories of planet formation to a strict test."

Scientists in 1995 discovered the first "hot Jupiter"-in other words, an extrasolar gas-giant planet with an orbital period of three to nine days. Today, more than 20 such planets are known to orbit other stars. These planets are believed to form in a disk of gas and condensed matter at or beyond three astronomical units (three times the 93-million-mile distance between the sun and Earth).

A sufficient amount of solid material exists at three astronomical units to produce a core capable of capturing enough gas to form a giant planet. After formation, these planets are believed to migrate inward to their present very close orbits.

If the parent star is orbited by a close stellar companion, then its gravitational pull can significantly truncate a protoplanetary disk around the main star. In the case of HD 188753, the two stellar companions would truncate the disk around the main star to a radius of only 1.3 astronomical units, leaving no space for a planet to form.

"How that planet formed in such a complicated setting is very puzzling. I believe there is yet much to be learned about how giant planets are formed," says Konacki.

The research was funded by NASA

http://www.caltech.edu/

The UK's leading team of planet-hunting astronomers, the Wide Angle Search for Planets (WASP), have announced the discovery of three new planets.

These extra-solar planets were seen to pass in front of, or transit, their host star. Studying such planets outside of our Solar System allows scientists to investigate how planetary systems form. WASP is the first team to detect planets in both the Northern and Southern Hemisphere using this technique.

Exoplanet expert Dr. Pierre Maxted comments “The planets are known as ‘hot-Jupiters’ as they are similar to Jupiter but are so close to their parent star that they orbit it in less than two days. This means that these planets have a surface temperature of nearly 2000°C and so are unlikely to host life. But finding these planets is important as these stars could also host much smaller planets similar to Earth, although detecting these worlds will be much more difficult”.

The planets orbit around stars similar to our Sun that are located at a distance of 850 light-years away from the Earth. Two are in the constellation of Phoenix visible only from the Southern hemisphere, while the third is in the Northern constellation of Lyra. All three stars are too faint to be seen with the naked eye, but are easily detectable with a small telescope.

Dr Coel Hellier, of Keele University, comments "When we see a transit we can deduce the size and mass of the planet and also what it is made of, so we can use these planets to study how solar systems form."

WASP-4 and WASP-5 are the first planets discovered by the WASP project's cameras in South Africa, and were confirmed by a collaboration with Swiss and French astronomers. "These two are now the brightest transiting planets in the Southern hemisphere" said Dr Hellier. WASP-3 is the third planet that the team has found in the North, using the SuperWASP camera sited in the Canary Islands.

Using data produced by SuperWASP’s cameras, which monitor up to 400,000 stars every minute, the new extra-solar planets were discovered as they were seen to pass in front of their host star.

Explaining the discovery, Dr Don Pollacco of Queen’s University, Belfast, Astrophysics Research Centre said: “We take pictures of the sky and measure the brightness of stars. If a planet is going around one of these stars and it happens to pass across the face of that star, our cameras will pick up the light from the star getting a little fainter.

“Discoveries such as these open up a whole new area of astronomy. Such transiting planets are important because they are the only ones that can have their mass and size measured directly. Astronomers can determine what they are made of and armed with this information we can begin to understand how these solar systems were formed.”

The WASP project is the most ambitious project in the world designed to discover large planets. Funding for the project comes from the UK Universities and the Science and Technology Facilities Council.

http://www.keele.ac.uk/

Boring” light from red dwarf star Gliese 581 means better odds for extraterrestrial life in that planetary system, according to University of British Columbia astronomer Jaymie Matthews.

Approximately 20.5 light years from the Earth, Gliese 581 made the headlines in April 2007 when European scientists discovered a planet, named Gliese 581c. Dubbed “SuperEarth,” the planet orbits Gliese 581 and could have water -- and thus able to support life.

“The Gliese 581 system is the first to be found -- beyond our own Earth -- that might have a liveable planet,” said Matthews.

Using Canada Space Agency’s suitcase-sized space telescope, the Microvariability and Oscillations of STars (MOST), Matthews put Gliese 581 on a six-week scientific stakeout following the April discovery. He will present his findings today at the Canadian Astronomical Society’s annual meeting in Kingston, Ontario.

Matthews and his team searched for the subtle dips in the light from the star when the planet’s orbit carried it directly between the star and the Earth, resulting in a “mini-eclipse” every 13 days. The depth of the dips would help researchers determine the size of the planet Gliese 581c, while the behaviour of the starlight at other times would help astronomers gauge the suitability of Gliese 581 as a “home star,” a star able to sustain life on planets around it.

“Gliese 581 seems remarkably stable over the six weeks it was monitored by MOST,” said Matthews. “The brightness of the star changed by only a few tenths of a percent over that time. This level of stability means that it provides a stable source of light -- hence heat -- to the surface of planet Gliese 581c.

“The climate there should not be a wild rollercoaster ride that would make it difficult for life to get a foothold,” said Matthews. “It also suggests the star is quite old, and settled in its ways, and that the planets around it have probably been around for billions of years.”

It took approximately 3.5 billion years for life on Earth to reach the level of complexity that we call human, said Matthews. “So if Gliese 581 has been around for at least that long, it’s more encouraging for the prospects of complex life on any planet around it.”

With space missions like MOST, the French satellite COROT, which joined MOST in orbit late last December, and the American Kepler mission due for launch in November 2008, Matthews predicts that other ‘Earthy’ worlds will come to light in the coming months and years.

“Some of them will have orbits that produce planetary alignments,” said Matthews. “Not the kind that excites somebody reading a horoscope but the kind that’s exciting for astronomers because they will allow us to test our models of alien worlds -- worlds that might be homes to neighbours in our Galactic city, the Milky Way.”

MOST is a Canadian Space Agency mission, jointly operated by Dynacon Inc., the University of Toronto Institute for Aerospace Studies and the University of British Columbia, with the assistance of the University of Vienna.

http://www.astro.ubc.ca/MOST/

Exoplanet researchers have discovered the lightest exoplanet found so far. The planet, “e”, in the famous system Gliese 581, is only about twice the mass of our Earth. The team also refined the orbit of the planet Gliese 581 d, first discovered in 2007, placing it well within the habitable zone, where liquid water oceans could exist.
These amazing discoveries are the outcome of more than four years of observations using the most successful low-mass-exoplanet hunter in the world, the HARPS spectrograph attached to the 3.6-metre ESO telescope at La Silla, Chile.

“The holy grail of current exoplanet research is the detection of a rocky, Earth-like planet in the ‘habitable zone’ — a region around the host star with the right conditions for water to be liquid on a planet’s surface,” says Michel Mayor from the Geneva Observatory, who led the European team to this stunning breakthrough.

Planet Gliese 581 e orbits its host star – located only 20.5 light-years away in the constellation Libra (“the Scales”) — in just 3.15 days. “With only 1.9 Earth-masses, it is the least massive exoplanet ever detected and is, very likely, a rocky planet”, says co-author Xavier Bonfils from Grenoble Observatory.

Being so close to its host star, the planet is not in the habitable zone. But another planet in this system appears to be. From previous observations — also obtained with the HARPS spectrograph at ESO’s La Silla Observatory and announced two years ago — this star was known to harbour a system with a Neptune-sized planet and two super-Earths. With the discovery of Gliese 581 e, the planetary system now has four known planets, with masses of about 1.9 (planet e), 16 (planet b), 5 (planet c), and 7 Earth-masses (planet d). The planet furthest out, Gliese 581 d, orbits its host star in 66.8 days.

“Gliese 581 d is probably too massive to be made only of rocky material, but we can speculate that it is an icy planet that has migrated closer to the star,” says team member Stephane Udry. The new observations have revealed that this planet is in the habitable zone, where liquid water could exist. “‘d’ could even be covered by a large and deep ocean — it is the first serious 'water world' candidate,” continued Udry.

The gentle pull of an exoplanet as it orbits the host star introduces a tiny wobble in the star’s motion — only about 7 km/hour, corresponding to brisk walking speed — that can just be detected on Earth with today’s most sophisticated technology. Low-mass red dwarf stars such as Gliese 581 are potentially fruitful hunting grounds for low-mass exoplanets in the habitable zone. Such cool stars are relatively faint and their habitable zones lie close in, where the gravitational tug of any orbiting planet found there would be stronger, making the telltale wobble more pronounced. Even so, detecting these tiny signals is still a challenge, and the discovery of Gliese 581 e and the refinement of Gliese 581 d’s orbit were only possible due to HARPS’s unique precision and stability.

“It is amazing to see how far we have come since we discovered the first exoplanet around a normal star in 1995 — the one around 51 Pegasi,” says Mayor. “The mass of Gliese 581 e is 80 times less than that of 51 Pegasi b. This is tremendous progress in just 14 years.”

The astronomers are confident that they can still do better. “With similar observing conditions an Earth-like planet located in the middle of the habitable zone of a red dwarf star could be detectable,” says Bonfils. “The hunt continues.”

http://www.eso.org/public/