by Kristen Minogue

18 November 2010

from ScienceMagazine Website

Improbable world.

An artist's depiction of a planet like the one discovered,

caught in the orbit of an old, violent star.
 

Astronomers have detected what they believe to be a planet at least the size of Jupiter that came from another galaxy.

If true, the world is the first planetary immigrant ever detected in the Milky Way. The find would also violate the leading hypothesis of how and where planets form.

The planet lives 2200 light-years away inside the Helmi stream, a ring of ancient stars that cuts through the plane of the Milky Way. Astronomers believe the stream formed 6 billion to 9 billion years ago, when the Milky Way ripped another galaxy to shreds, swallowing some of its stars in the process.

Astronomer Johny Setiawan of the Max Planck Institute for Astronomy in Heidelberg, Germany, likes looking at these stars because they tend to have unusual properties.

But even by these standards, one star in particular caught his eye: HIP 13044.

Setiawan and colleagues at Max Planck and the European Space Agency noticed that the star didn't move at a constant velocity relative to our sun. Their instruments picked up a regular 16-day pattern, where sometimes the star was closer and sometimes farther away. That could be caused by spots on the star, which could confuse velocity measurements, or by the star expanding and contracting (known as stellar pulsations).

But the researchers didn't detect either. That left only one possibility: a planet gravitationally pulling on the star.

If that's the case, the world is the first foreign planet detected in the Milky Way, the team reports online today in Science. But that's not the only unusual thing about it.

It shouldn't have formed in the first place.

That's because HIP 13044 is a very ancient, very metal-poor star. It's about 3 billion years older than our own sun and has only 1% as much metal.

Until now, the prevailing hypothesis has said that as stars evolve, metals (astronomers' term for any chemical elements heavier than hydrogen and helium) in the swirling disk around them form tiny "seeds" that attract other matter and slowly grow into planets.

So far most surveys have backed up the theory: Stars rich in metals, such as our Sun, are much more likely to have planets than stars that don't.

"The fact that you can form planets around a star that has so little of this material is a very surprising and unusual thing," says Christopher Johns-Krull, an astronomer at Rice University in Houston, Texas, who was not involved in the new work.

But there is a second, more violent, theory about how planets form.

If the swirling disk of gas is massive enough - one-tenth the mass of its star or more - Johns-Krull says the gravitational power of the disk can make the disk unstable. In those cases, the star isn't big enough to keep the disk in check, so the disk starts to collapse under its own gravity and forms planets.

Although it's still not certain that a planet is behind the variations in the star's speed, all the findings so far point that way, says NASA astronomer Steven Pravdo. To be sure, astronomers will need to run other tests.

Ideally, they would look for the planet to pass in front of the star, but that would work only if the planet orbits at just the right angle, he says.

"The claim that it's extragalactic is kind of a guess," says Pravdo.

It's possible the star was originally part of the Milky Way when it collided with the other galaxy billions of years ago.

Although an extragalactic planet,

"is a nice possibility," says Pravdo, the more exciting find is what this planet could do to the idea of tame, gradual planet formation.

Johns-Krull agrees.

"This planet says, maybe that's not right," he says. "Maybe it's this other, more dramatic process."

 

'Alien' Planet Detected Circling Dying Star
by Neil Bowdler

Science reporter, BBC News

18 November 2010

from BBC Website

This artist's impression shows HIP 13044 b,

an exoplanet orbiting a star that entered our galaxy, the Milky Way, from another galaxy

This artist's impression shows HIP 13044 b,

an exoplanet orbiting a star that entered the Milky Way from another galaxy
 

Astronomers claim to have discovered the first planet originating from outside our galaxy.

The Jupiter-like planet, they say, is part of a solar system which once belonged to a dwarf galaxy. This dwarf galaxy was in turn devoured by our own galaxy, the Milky Way, according to a team writing in the academic journal Science.

The star, called HIP 13044, is nearing the end of its life and is 2000 light years from Earth.

The discovery was made using a telescope in Chile.
 

Cosmic cannibalism

Planet hunters have so far netted nearly 500 so-called "exoplanets" outside our Solar System using various astronomical techniques.

But all of those so far discovered, say the researchers, are indigenous to our own galaxy, the Milky Way.

This find is different, they say, because the planet circles a sun which belongs to a group of stars called the "Helmi stream" which are known to have once belonged to a separate dwarf galaxy.

This galaxy was gobbled up by the Milky Way between six and nine billion years ago in an act of intergalactic cannibalism.

The new planet is thought to have a minimum mass 1.25 times that of Jupiter and circles in close proximity to its parent star, with an orbit lasting just 16.2 days.

The exoplanet was detected by a European team of astronomers

using the MPG/ESO 2.2-meter telescope at ESO's La Silla Observatory in Chile

The exoplanet was detected by a team using the MPG/ESO 2.2-m telescope in Chile
 

It sits in the southern constellation of Fornax.

The planet would have been formed in the early era of its solar system, before the world was incorporated into our own galaxy, say the researchers.

"This discovery is very exciting," said Rainer Klement of the Max Planck Institute for Astronomy in Heidelberg, Germany, who targeted the stars in the study.

"For the first time, astronomers have detected a planetary system in a stellar stream of extragalactic origin. This cosmic merger has brought an extragalactic planet within our reach."

Dr Robert Massey of the UK's Royal Astronomical Society said the paper provided the first "hard evidence" of a planet of extragalactic origin.

"There's every reason to believe that planets are really quite widespread throughout the Universe, not just in our own galaxy, the Milky Way, but also in the thousands of millions of others there are," he said, "but this is the first time we've got hard evidence of that."

End Days

The new find might also offer us a glimpse of what the final days of our own Solar System may look like.

HIP 13044 is nearing its end. Having consumed all the hydrogen fuel in its core, it expanded massively into a "red giant" and might have eaten up smaller rocky planets like our own Earth in the process, before contracting.

The new Jupiter-like planet discovered appears to have survived the fireball, for the moment.

"This discovery is particularly intriguing when we consider the distant future of our own planetary system, as the Sun is also expected to become a red giant in about five billion years," said Dr Johny Setiawan, who also works at the Max Planck Institute for Astronomy, and who led the study.

"The star is rotating relatively quickly," he said. "One explanation is that HIP 13044 swallowed its inner planets during the red giant phase, which would make the star spin more quickly."

The new planet was discovered using what is called the "radial velocity method" which involves detecting small wobbles in a star caused by a planet as it tugs on its sun.

These wobbles were picked up using a ground-based telescope at the European Southern Observatory's La Silla facility in Chile.

Extragalactic Exoplanet

...Found Hiding Out in Milky Way
by Ron Cowen
Science News Email Author
November 18, 2010

from Wired Website

Some extrasolar planets are truly out of this world.
 

Astronomers have for the first time discovered a planet in the Milky Way that came from another galaxy. The planet, which has a mass of at least 1.25 Jupiters, orbits an elderly star that was ripped from a small satellite galaxy some 6 to 9 billion years ago.

Johny Setiawan and Rainer Klement of the Max Planck Institute for Astronomy in Heidelberg, Germany, describe the finding online November 18 in Science.

“The coolness factor is definitely that the planet and star came from another galaxy,” says Sara Seager of MIT, who was not part of the study. “The planet almost certainly formed during the time the star was in the other galaxy.”

In hunting for extrasolar planets, Setiawan and his colleagues homed in on HIP 13044, about 2,000 light-years from Earth, because it’s part of a stream of stars called Helmi, believed to have originated in another galaxy.

The star’s motion could also be monitored for many months each year with a spectrograph at the European Southern Observatory’s La Silla site in Chile, looking for telltale wobbles that would indicate the tiny tug of an unseen, orbiting planet.

HIP 13044 and the other stars in the Helmi stream stand out in the solar neighborhood because they have elongated orbits that take them about 42,000 light-years above and below the plane of the Milky Way’s disk. Such orbits strongly suggest the stars were part of a group torn from a satellite galaxy and stretched out by gravitational tidal forces into a filament or stream.

The discovery, notes planet hunter Scott Gaudi of Ohio State University in Columbus,

“is doubly weird: It is a weird planet around a weird star.”

The star is unusual because it has the lowest abundance of metals - about 1 percent of the sun’s - of any star known to have a planet. (In astronomical parlance, a metal refers to any element heavier than helium.)

The vast majority of the roughly 500 extrasolar planets known are found around stars with a much higher metal abundance, and the leading theory of planet formation suggests that stars with high metal contents are those that form giant, Jupiter-like planets.

Also unusual is that HIP 13044 is old enough to have exhausted its supply of hydrogen fuel and passed through the red giant phase of evolution, in which it mushroomed in size. Since then the star contracted to a diameter about seven times that of the sun and is now burning helium at its core. A star in this phase of evolution, known as the red horizontal branch, has never before been found to have a planet.

In part, that’s because the enhanced activity of old, evolved stars, including the presence of magnetically driven disturbances known as starspots, makes it more difficult to discern a stellar wobble, says Setiawan.

In addition,

“there is a high risk that you will not find any planets because they have been engulfed by the star during the [red giant] evolutionary phase,” he adds.

In order to survive, HIP 13044’s planet, which now resides much closer to the star than Mercury does to the sun, must have originally orbited at a much greater distance, the researchers say.

That’s the only way it could have escaped being swallowed during the time the star was a red giant. (In several billion years, the sun will also become a puffed-up red giant and is likely to engulf Earth and the other inner planets.)

Other planets that resided closer to HIP 13044 would not have been so lucky.

One explanation for the star’s relatively rapid rate of rotation is that it has been spun up by the angular momentum of planets it swallowed. Other rapidly rotating, elderly stars that have evolved to the red horizontal branch may have had similar dining habits, researchers have previously noted.

Even though the newfound planet has dodged one bullet, it will soon face another. In a few million years, when the star exhausts all the helium forged at its core, it will undergo a more rapid and larger expansion in which the planet is likely to be destroyed.

Planet From Another Galaxy Discovered
by Best0fScience
November 18, 2010

from YouTube Website

First planet of extragalactic origin

An exoplanet orbiting a star that entered our galaxy, the Milky Way, from another galaxy has been detected by a European team of astronomers using the MPG/ESO 2.2-metre telescope at ESO's La Silla Observatory in Chile.

The Jupiter-like planet is particularly unusual, as it is orbiting a star nearing the end of its life and could be about to be engulfed by it, giving clues about the fate of our own planetary system in the distant future.

Astronomers have detected nearly 500 planets orbiting stars in our cosmic neighborhood, but none outside our Milky Way has been confirmed. Now, however, a planet weighing at least 1.25 times as much as Jupiter has been discovered orbiting a star of extragalactic origin, even though the star now finds itself within our own galaxy.

The star, which is known as HIP 13044, lies about 2000 light-years from Earth and is part of the so-called Helmi stream. This stream of stars originally belonged to a dwarf galaxy, which was devoured by our Milky Way in an act of galactic cannibalism six to nine billion years ago.

Astronomers detected the planet by looking for tiny telltale wobbles of the star caused by the gravitational tug of an orbiting companion. For these precise observations, the team used a high-resolution spectrograph called FEROS, attached to the 2.2-metre telescope at ESO's La Silla Observatory in Chile.

The planet, HIP 13044 b, is also one of the few exoplanets known to have survived its host star massively growing in size after exhausting the hydrogen fuel supply in its core - the Red Giant phase of stellar evolution.

HIP 13044 b is near to its host star. At the closest point in its elliptical orbit, it is less than one stellar diameter from the surface of the star (or 0.055 times the Sun-Earth distance). and completes an orbit in only about 16 days. The astronomers hypothesize that the planet's orbit might initially have been much larger, but that it moved inwards during the Red Giant phase.

Any closer-in planets may not have been so lucky. Astronomers suggest that some inner planets may have been swallowed by the star during the Red Giant phase.

Although the Jupiter-like exoplanet has escaped the fate of these inner planets so far, the star will expand again in the next stage of its evolution. When this happens, the star may engulf the planet, meaning it may be doomed after all.

The astronomers are now searching for more planets around stars near the ends of their lives.

Their work may tell us about the fate of planets in the distant future of our own Solar System, as the Sun is also expected to become a Red Giant in about five billion years.