Time Space Science

Scientists using a combination of ground-based and orbiting telescopes have discovered a failed star, less than one-hundredth the mass of the sun, possibly in the process of forming a solar system. It is the smallest known star-like object to harbor what appears to be a planet-forming disk of rocky and gaseous debris, which one day could evolve into tiny planets and create a solar system in miniature.


A team led by Kevin Luhman, assistant professor of astronomy and astrophysics at Penn State, will discuss this finding in the Dec. 10 issue of Astrophysical Journal Letters.

The discovered object, called a brown dwarf, is described as a "failed star" because it is not massive enough to sustain nuclear fusion like our sun. The object is only eight times more massive than Jupiter. The fact that a brown dwarf this small could be in the midst of creating a solar system challenges the very definition of star, planet, moon and solar system.

"Our goal is to determine the smallest 'sun' with evidence for planet formation," said Luhman. "Here we have a sun that is so small it is the size of a planet. The question then becomes, what do we call any little bodies that might be born from this disk: planets or moons?" If this protoplanetary disk does form into planets, the whole system would be a miniaturized version of our solar system -- with the central "sun", the planets and their orbits all roughly 100 times smaller.

Luhman's team detected the brown dwarf, called Cha 110913-773444, with NASA's Spitzer Space Telescope, the Hubble Space Telescope and two telescopes in the Chilean Andes, the Blanco telescope of the Cerro Tololo Inter-American Observatory and the Gemini South telescope, both international collaborations funded in part by the National Science Foundation. Luhman led a similar observation last year that uncovered a 15-Jupiter-mass brown dwarf with a protoplanetary disk.

Brown dwarfs are born like stars, condensing out of thick clouds of gas and dust. But unlike stars, brown dwarfs do not have enough mass -- and therefore do not have enough pressure and temperature in their cores -- to sustain nuclear fusion. They remain relatively cool objects visible in lower-energy wavelengths such as infrared. A protoplanetary disk is a flat disk made up of dust and gas that is thought to clump together to form planets. Our solar system was formed from such a disk about 5 billion years ago. NASA's Spitzer telescope has found dozens of disk-sporting brown dwarfs so far, several of which show the initial stages of the planet-building process. The material in these disks is beginning to stick together into what may be the "seeds" of planets.

With Spitzer, the science team spotted Cha 110913-773444 about 500 light years away in the constellation Chamaeleon. This brown dwarf is young, only about 2 million years old. The team studied properties of the brown dwarf with infrared instruments on the other observatories. The cool, dim protoplanetary disk was detectable only with Spitzer's Infrared Array Camera, which was developed at the Harvard-Smithsonian Center for Astrophysics.

In the past decade, advances in astronomy have led to the detection of small brown dwarfs and massive extra-solar planets, which has brought about a quandary in taxonomy. "There are two camps when it comes to defining planets versus brown dwarfs," said team member Giovanni Fazio of the Harvard-Smithsonian Center for Astrophysics. "Some go by size, and others go by how the object formed. For instance, this new object would be called a planet based on its size, but a brown dwarf based on how it formed." If one were to call the object a planet, Fazio said, then Spitzer may have discovered its first "moon-forming" disk. No matter what the final label may be, one thing is clear: The universe produces some strange solar systems very different from our own. Other members of the discovery team are Lucia Adame and Paola D'Alessio of the National Autonomous University of Mexico and Nuria Calvet and Lee Hartmann of the University of Michigan.

The 4-meter Blanco Telescope at the Cerro Tololo Inter-American Observatory in Chile is part of the National Optical Astronomy Observatory, which is operated by the Association of Universities for Research in Astronomy (AURA) Inc. under a cooperative agreement with the National Science Foundation. The nearby 8-meter Gemini South telescope also is managed by AURA. NASA's Goddard Space Flight Center, Greenbelt, Md., built Spitzer's Infrared Array Camera. The instrument's principal investigator is Giovanni Fazio. The Jet Propulsion Laboratory, Pasadena, Calif., manages the Spitzer mission for NASA. Science operations are conducted at the Spitzer Science Center at the California Institute of Technology in Pasadena.

http://www.psu.edu/

An international team of astronomers have accurately determined the radius and mass of the smallest core-burning star known until now.
The observations were performed in March 2004 with the FLAMES multi-fibre spectrograph on the 8.2-m VLT Kueyen telescope at the ESO Paranal Observatory (Chile). They are part of a large programme aimed at measuring accurate radial velocities for sixty stars for which a temporary brightness "dip" has been detected during the OGLE survey.

The astronomers find that the dip seen in the light curve of the star known as OGLE-TR-122 is caused by a very small stellar companion, eclipsing this solar-like star once every 7.3 days.

This companion is 96 times heavier than planet Jupiter but only 16% larger. It is the first time that direct observations demonstrate that stars less massive than 1/10th of the solar mass are of nearly the same size as giant planets. This fact will obviously have to be taken into account during the current search for transiting exoplanets.

In addition, the observations with the Very Large Telescope have led to the discovery of seven new eclipsing binaries, that harbour stars with masses below one-third the mass of the Sun, a real bonanza for the astronomers

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

Using the European Southern Observatory's Very Large Telescope in Chile, an international team of researchers discovered a brown dwarf belonging to the 24th closest stellar system to the Sun. Brown dwarfs are intermediate objects that are neither stars nor planets. This object is the third closest brown dwarf to the Earth yet discovered, and one of the coolest, having a temperature of about 750 degrees Centigrade. It orbits a very small star at about 4.5 times the mean distance between the Earth and the Sun. Its mass is estimated to be somewhere between 9 and 65 times the mass of Jupiter.


At a time when astronomers are peering into the most distant Universe, looking at objects as far as 13 billion light-years away, one may think that our close neighbourhood would be very well known. Not so. Astronomers still find new star-like objects in our immediate vicinity. Using ESO's VLT, they just discovered a brown dwarf companion to the red star SCR 1845-6357, the 36th closest star to the Sun.

"This newly found brown dwarf is a valuable object because its distance is well known, allowing us to determine with precision its intrinsic brightness", said team member Markus Kasper (ESO). "Moreover, from its orbital motion, we should be able in a few years to estimate its mass. These properties are vital for understanding the nature of brown dwarfs."

To discover this brown dwarf, the team used the high-contrast adaptive optics NACO Simultaneous Differential Imager (SDI) on ESO's Very Large Telescope, an instrument specifically developed to search for extrasolar planets. The SDI camera enhances the ability of the VLT and its adaptive optics system to detect faint companions that would normally be lost in the glare of the primary star. In particular, the SDI camera provides additional, often very useful spectral information which can be used to determine a rough temperature for the object without follow-up observations.

Located 12.7 light-years away from us, the newly found object is nevertheless not the closest brown dwarf. This honour goes indeed to the two brown dwarfs surrounding the star Epsilon Indi, located 11.8 light years away.

However, this newly discovered brown dwarf is unique in many aspects. "Besides being extremely close to Earth, this object is a T dwarf - a very cool brown dwarf - and the only such object found as a companion to a low-mass star," said Beth Biller, a graduate student at the University of Arizona and lead author of the paper reporting the discovery. "It is also likely the brightest known object of its temperature because it is so close."

The discovery of this brown dwarf hints that, at least close to the Sun, cool brown dwarfs prefer to be part of a couple with a star or another brown dwarf, rather than wandering alone in the cosmic emptiness. Indeed, of the seven cool brown dwarfs that reside within 20 light years of the Sun, five have a companion.

"This has wide-ranging implications for theories of brown dwarf formation, which, until now, tend to favour the production of single brown dwarfs," said team member Laird Close (University of Arizona).

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

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/

The first detailed images of a binary asteroid system reveal a bizarre world where the highest points on the surface are actually the lowest, and the two asteroids dance in each other's gravitational pull.

A binary asteroid is a system where two asteroids orbit around one another, like a mini Earth-moon system, said Daniel Scheeres, University of Michigan associate professor of aerospace engineering. The new results are scheduled to appear Oct. 12 in the journal Science in a pair of papers by Scheeres and Dr. Steven Ostro of the NASA/Caltech Jet Propulsion Laboratory.

The radar images of asteroid KW4 (the official full designation is 66391 1999 KW4) were obtained in May 2001, when the asteroid passed 4.8 million kilometers from Earth. Previously, KW4 was classified as a potentially hazardous asteroid (PHA) because of the proximity of the asteroid's orbit to Earth's orbit. The new observations show that there is no chance of KW4 hitting Earth within at least the next 1,000 years, Scheeres said.

"The KW4 results have profound consequences for ideas about mitigation of the asteroid collision hazard," Scheeres said.

The observations show that the larger object is spinning in its orbit so fast that it has been flattened into a kind of flying saucer shape, said Scheeres. Because of this, the mountainous region along the center of the asteroid actually forms the lowest part on the asteroid. In fact the asteroid is spinning so fast that the equatorial ridge is very close to lifting off the surface and spinning into space, he said.

Another interesting finding is that the two bodies in the asteroid system are orbiting so closely that they are caught in each other's gravitational pull.

"They are so close together that when one rotates it affects the other's movements," Scheeres said.

Based on the observations, the KW4 binary asteroid appears to have formed either from tidal disruption during a close pass by the Earth or from sunlight shining on it, so that it spins so fast that it eventually broke into two pieces. The odd shapes of asteroids cause them to sometimes spin faster and faster when illuminated by the sun, acting a bit like a solar sail, Scheeres said. This is called the YORP effect.

The recent findings also confirm that the asteroids are only floating piles of rubble held together by gravity and not a solid mass.


http://www.umich.edu/

A new study published in Nature this week reveals that asteroid surfaces age and redden much faster than previously thought — in less than a million years, the blink of an eye for an asteroid. This study has finally confirmed that the solar wind is the most likely cause of very rapid space weathering in asteroids.
This fundamental result will help astronomers relate the appearance of an asteroid to its actual history and identify any after effects of a catastrophic impact with another asteroid.

“Asteroids seem to get a ‘sun tan’ very quickly,” says lead author Pierre Vernazza. “But not, as for people, from an overdose of the Sun’s ultraviolet radiation, but from the effects of its powerful wind.”

It has long been known that asteroid surfaces alter in appearance with time — the observed asteroids are much redder than the interior of meteorites found on Earth [1] — but the actual processes of this “space weathering” and the timescales involved were controversial.

Thanks to observations of different families of asteroids [2] using ESO’s New Technology Telescope at La Silla and the Very Large Telescope at Paranal, as well as telescopes in Spain and Hawaii, Vernazza’s team have now solved the puzzle.

When two asteroids collide, they create a family of fragments with “fresh” surfaces. The astronomers found that these newly exposed surfaces are quickly altered and change colour in less than a million years — a very short time compared to the age of the Solar System.

“The charged, fast moving particles in the solar wind damage the asteroid’s surface at an amazing rate [3]”, says Vernazza. Unlike human skin, which is damaged and aged by repeated overexposure to sunlight, it is, perhaps rather surprisingly, the first moments of exposure (on the timescale considered) — the first million years — that causes most of the aging in asteroids.

By studying different families of asteroids, the team has also shown that an asteroid’s surface composition is an important factor in how red its surface can become. After the first million years, the surface “tans” much more slowly. At that stage, the colour depends more on composition than on age. Moreover, the observations reveal that collisions cannot be the main mechanism behind the high proportion of “fresh” surfaces seen among near-Earth asteroids. Instead, these “fresh-looking” surfaces may be the results of planetary encounters, where the tug of a planet has “shaken” the asteroid, exposing unaltered material.

Thanks to these results, astronomers will now be able to understand better how the surface of an asteroid — which often is the only thing we can observe — reflects its history.

Notes

[1] Meteorites are small fragments of asteroids that fall on Earth. While a meteorite enters the Earth's atmosphere its surface can melt and be partially charred by the intense heat. Nevertheless, the meteorite interior remains unaffected, and can be studied in a laboratory, providing a wealth of information on the nature and composition of asteroids.

[2] An asteroid family is a group of asteroids that are on similar orbits around the Sun. The members of a given family are believed to be the fragments of a larger asteroid that was destroyed during a collision.

[3] The surface of an asteroid is affected by the highly energetic particles forming the solar wind. These particles partially destroy the molecules and crystals on the surface, re-arranging them in other combinations. Over time, these changes give formation of a thin crust or irradiated material with distinct colours and properties.

http://www.nature.com/nature/journal/v458/n7241/full/nature07956.html