Planet Or Failed Star? One Of Smallest Stellar Companions Seen By Hubble
The Hubble observation of the diminutive companion to the low-mass red dwarf star CHRX 73 is a dramatic reminder that astronomers do not have a consensus in deciding which objects orbiting other stars are truly planets - even though they have recently provided the definition of 'planet' for objects inside our Solar System.
Kevin Luhman of Penn State University, USA, leader of the international team that found the object (called CHRX 73 'B') is casting his vote for a brown dwarf. "New, more sensitive telescopes are finding smaller and smaller objects of planetary-mass size," said Luhman. "These discoveries have prompted astronomers to ask the question, are planetary-mass companions always planets?"
Some astronomers suggest that an extra-solar object's mass determines whether it is a planet. Luhman and others advocate that an object is only a planet if it formed from the disk of gas and dust that commonly encircles a newborn star. Our Solar System planets formed 4.6 thousand million years ago out of a dust disk around our Sun.
Brown dwarfs, by contrast, form just like stars: from the gravitational collapse of large, diffuse clouds of hydrogen gas. Unlike stars, brown dwarfs do not have quite enough mass to ignite hydrogen fusion reactions in their cores, which power stars such as our Sun.
CHXR 73 B is 31.2 thousand million kilometres from its red dwarf sun. This is roughly 200 times farther than Earth is from our Sun. Being about two million years old, the star is very young when compared with our middle-aged 4.6-thousand-million-year-old Sun.
"The object is so far away from its star that it is unlikely to have formed in a circumstellar disk," Luhman explained. Disks around low-mass stars are about 8 to 16 thousand million kilometres in diameter. There isn't enough material at that distance from the red dwarf to create a planet. Theoretical models show that giant planets like Jupiter form no more than about 5 thousand million kilometres from their stars.
Hubble's Advanced Camera for Surveys discovered the object while conducting a survey of free-floating brown dwarfs. Astronomers have found hundreds of brown dwarfs in our galaxy since the first brown dwarfs were spied about a decade ago. Most of them are floating through space and not orbiting stars.
"The study of sub-stellar objects in orbit around a star allows us to determine the age, and over time also the mass of the companion. Such studies help us to improve our understanding of the formation and inner structure of brown dwarfs and planets," says Wolfgang Brandner of the Max-Planck-Institute for Astronomy in Heidelberg, Germany.
One way to further settle the uncertainty would be if a disk of dust could be observed around CHRX 73's companion. Like stars, brown dwarfs have circumstellar disks, too. They would be no more than about 4 thousand million kilometres in diameter.
NASA's Spitzer Space Telescope has detected disks around several free-floating brown dwarfs. But CHRX 73 B is too close to its star for Spitzer to detect the disk. So astronomers will have to wait for the launch of the NASA/ESA/CSA James Webb Space Telescope in 2013 to determine if this companion has a disk. The Webb telescope will combine Hubble's sharpness, which is needed for detecting close companions, and Spitzer's infrared sensitivity, which is necessary for seeing cool, dusty disks.
There is also a roughly 0.1 percent probability that CHRX 73 B is a background object that by chance happens to align with CHRX 73.The team's results will appear in the 20 September 2006 issue of the Astrophysical Journal.
Members of the research team are K. L. Luhman, Penn State University, USA; J. C. Wilson, M. F. Skrutskie, M. J. Nelson, and D. E. Peterson, University of Virginia, USA; W. Brandner, Max-Planck Institute for Astronomy, Germany; and J. D. Smith, M. C. Cushing, and E. Young, University of Arizona, USA.
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
Astronomers Confirm The First Image Of A Planet Outside Of Our Solar System
In February and March of this year, the astronomers took new images of the young brown dwarf and its giant planet companion with the state-of-the-science NACO instrument on the European Southern Observatory's (ESO) Very Large Telescope in northern Chile. The planet is near the southern constellation of Hydra and approximately 200 light years from Earth.
"Our new images show convincingly that this really is a planet, the first planet that has ever been imaged outside of our solar system," said Gael Chauvin, astronomer at the ESO and leader of the team of astronomers who conducted the study.
"The two objects — the giant planet and the young brown dwarf — are moving together; we have observed them for a year, and the new images essentially confirm our 2004 finding," said Benjamin Zuckerman, UCLA professor of physics and astronomy, member of NASA's Astrobiology Institute, and a member of the team. "I'm more than 99 percent confident. This is also the first time that a planet outside of our solar system has been detected far from a star or brown dwarf — nearly twice as far as the distance between Neptune and the sun."
Anne-Marie Lagrange, another member of the team from the Grenoble Observatory in France, said, "Our discovery represents a first step towards one of the most important goals of modern astrophysics: to characterize the physical structure and chemical composition of giant and, eventually, terrestrial-like planets."
Last September, the same team of astronomers reported a faint reddish speck of light in the close vicinity of a young brown dwarf. The feeble object, now called 2M1207b, is more than 100 times fainter than the brown dwarf, 2M1207A. The spectrum of 2M1207b presents a strong signature of water molecules, thereby confirming that it must be cold. Based on the infrared colors and the spectral data, evolutionary model calculations led to the conclusion that 2M1207b is a five-Jupiter-mass planet. Its mass can be estimated also by use of a different method of analysis, which focuses on the strength of its gravitational field; this technique suggests that the mass might be even less than that of five Jupiters.
At the time of its discovery in April 2004, it was impossible to prove that the faint source is not a background object (such as an unusual galaxy or a peculiar cool star with abnormal infrared colors), even though this appeared very unlikely. Observations with the Hubble Space Telescope, obtained in August 2004, corroborated the VLT/NACO observations, but were taken too soon after the NACO ones to demonstrate conclusively that the faint source is a planet.
The new observations show with high confidence that the two objects are moving together and hence are gravitationally bound.
The paper describing this research has been accepted for publication in Astronomy and Astrophysics, a premier journal in astronomy.
"Given the rather unusual properties of the 2M1207 system, the giant planet most probably did not form like the planets in our solar system," Chauvin said. "Instead it must have formed the same way our sun formed, by a one-step gravitational collapse of a cloud of gas and dust."
The same European/American team has had another paper just accepted for publication in Astronomy and Astrophysics. This paper reports the imaging discovery with the same VLT/NACO instrumentation of a lightweight companion to AB Pictoris, a young star located about 150 light years from Earth. The estimated mass of the companion is between 13 and 14 times the mass of Jupiter, which places the companion right on the borderline between massive planets and the lowest mass brown dwarfs.
"Remarkably, this companion is located very far from its host star — about nine times farther from AB Pictoris than Neptune is from the sun," Zuckerman said. Nothing so far from its star has ever been seen in a planetary system before, he added.
Brown dwarfs, the missing link between gas giant planets like Jupiter and small, low‑mass stars, are failed stars about the size of Jupiter, with a much larger mass — but not quite large enough to become stars. Like the sun and Jupiter, they are composed mainly of hydrogen gas, perhaps with swirling cloud belts. Unlike the sun, they cannot fuse protons to helium nuclei as their primary internal energy source, and they emit almost no visible light.
Web sites for reference:
ESO: http://www.eso.org/
UCLA Astronomy and Astrophysics: http://www.astro.ucla.edu
Scientists Find Possible Birth Of Tiniest Known Solar System
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.
Is It A Planet? Exotic Object Orbiting Star Stirs Exoplanet Classification Rethink
The object, named COROT-exo-3b, is about the size of Jupiter, but packs more than 20 times the mass. It takes only 4 days and 6 hours to orbit its parent star, which is slightly larger than the Sun.
COROT-exo-3b was found as the satellite observed the drop in the brightness of the star each time the object (COROT-exo-3b) passed in front. "We were taken by surprise when we found this massive object orbiting so close to its parent star", said Dr Magali Deleuil from the Laboratoire d'Astrophysique de Marseille (LAM), leader of the team that made the discovery. She added, "COROT-exo-3b is really unique - we’re still debating its nature."
The search for planets with orbital periods less than 10 days orbiting close to the parent star has lasted almost 15 years. During this time, scientists have encountered planets with masses 12 times that of Jupiter, and stars 70 times as massive as Jupiter, but none in between. This is why the 20-Jupiter-mass COROT-exo-3b was such a surprise.
This odd find does not fall into either conventional category of planets or brown dwarfs. A brown dwarf is a ‘failed star’, a sub-stellar object that is not undergoing nuclear fusion at its core, but displays some stellar characteristics.
"COROT-exo-3b might turn out to be a rare object found by sheer luck", said Dr Francois Bouchy, from Institut d'Astrophysique de Paris (IAP), member of the team that made the discovery. “But it might just be a member of a new-found family of very massive planets that encircle stars more massive than our Sun. We’re now beginning to think that the more massive the star, the more massive the planet," he said.
Team member Dr Hans Deeg, of the Instituto de Astrofisica de Canarias (IAC), explains why this new object is such an important find for planet hunters, "It has puzzled us; we’re not sure where to draw the boundary between planets and brown dwarfs.”
As a planet, COROT-exo-3b would be the most massive and the densest found to date - more than twice as dense as lead. Studying it will help them better understand how to categorise such objects. The team also wants to understand how such a massive object formed so close to its parent.
This discovery of COROT-exo-3b was supported by a number of ground-based observations that made use of a network of observatories operated by different institutes worldwide. The telescope of Observatoire de Haute Provence (France) studied the object’s mass, orbit and stellar properties; the European Southern Observatory telescopes at Paranal and La Silla (Chile), looked into the properties of its parent star; the Thuringia State Observatory in Tautenburg (Germany) was used to help determining the object’s mass and orbit; the Canada-France-Hawaii Telescope on Mauna Kea looked for signals from faint stars in the vicinity; the Swiss Euler Telescope at La Silla (Chile) helped determine its mass and orbit; the Wise Observatory (Israel), the ESA telescope on Mt. Teide, Tenerife, and the telescope of the Astrophysical Institute of the Canary Islands were used to exclude signal contributions from other stars.
The results are due to appear in ‘Transiting exoplanets from the COROT space mission, VI. COROT-exo-3b: The first secure inhabitant of the Brown-dwarf desert’ by M. Deleuil et al, in the journal Astronomy and Astrophysics.
COROT is a mission led by the French Space Agency (CNES), with contributions from ESA, Austria, Belgium, Germany, Spain and Brazil. It was launched in December 2006. It carries a 27-cm aperture telescope designed to detect tiny changes in brightness from nearby stars. The mission’s main objectives are to search for exoplanets and to study stellar interiors.
Turning Planetary Theory Upside Down: Nine New Exoplanets Found, Some With Retrograde Orbits
"This is a real bomb we are dropping into the field of exoplanets," says Amaury Triaud, a PhD student at the Geneva Observatory who, with Andrew Cameron and Didier Queloz, leads a major part of the observational campaign.
Planets are thought to form in the disc of gas and dust encircling a young star. This proto-planetary disc rotates in the same direction as the star itself, and up to now it was expected that planets that form from the disc would all orbit in more or less the same plane, and that they would move along their orbits in the same direction as the star's rotation. This is the case for the planets in the Solar System.
After the initial detection of the nine new exoplanets [1] with the Wide Angle Search for Planets (WASP, [2]), the team of astronomers used the HARPS spectrograph on the 3.6-metre ESO telescope at the La Silla observatory in Chile, along with data from the Swiss Euler telescope, also at La Silla, and data from other telescopes to confirm the discoveries and characterise the transiting exoplanets [3] found in both the new and older surveys.
Surprisingly, when the team combined the new data with older observations they found that more than half of all the hot Jupiters [4] studied have orbits that are misaligned with the rotation axis of their parent stars. They even found that six exoplanets in this extended study (of which two are new discoveries) have retrograde motion: they orbit their star in the "wrong" direction.
"The new results really challenge the conventional wisdom that planets should always orbit in the same direction as their stars spin," says Andrew Cameron of the University of St Andrews, who presented the new results at the RAS National Astronomy Meeting (NAM2010) in Glasgow this week.
In the 15 years since the first hot Jupiters were discovered, their origin has been a puzzle. These are planets with masses similar to or greater than that of Jupiter, but that orbit very close to their suns. The cores of giant planets are thought to form from a mix of rock and ice particles found only in the cold outer reaches of planetary systems. Hot Jupiters must therefore form far from their star and subsequently migrate inwards to orbits much closer to the parent star. Many astronomers believed this was due to gravitational interactions with the disc of dust from which they formed. This scenario takes place over a few million years and results in an orbit aligned with the rotation axis of the parent star. It would also allow Earth-like rocky planets to form subsequently, but unfortunately it cannot account for the new observations.
To account for the new retrograde exoplanets an alternative migration theory suggests that the proximity of hot Jupiters to their stars is not due to interactions with the dust disc at all, but to a slower evolution process involving a gravitational tug-of-war with more distant planetary or stellar companions over hundreds of millions of years. After these disturbances have bounced a giant exoplanet into a tilted and elongated orbit it would suffer tidal friction, losing energy every time it swung close to the star. It would eventually become parked in a near circular, but randomly tilted, orbit close to the star. "A dramatic side-effect of this process is that it would wipe out any other smaller Earth-like planet in these systems," says Didier Queloz of Geneva Observatory.
Two of the newly discovered retrograde planets have already been found to have more distant, massive companions that could potentially be the cause of the upset. These new results will trigger an intensive search for additional bodies in other planetary systems.
This research was presented at the Royal Astronomical Society National Astronomy Meeting (NAM2010) that is taking place this week in Glasgow, Scotland. Nine publications submitted to international journals will be released on this occasion, four of them using data from ESO facilities. On the same occasion, the WASP consortium was awarded the 2010 Royal Astronomical Society Group Achievement Award.
Baby Stars in the Rosette Cloud
The image is a new release of 'OSHI', ESA's Online Showcase of Herschel Images.
The Rosette Nebula resides some 5,000 light years from Earth and is associated with a larger cloud that contains enough dust and gas to make the equivalent of 10,000 Sun-like stars. The Herschel image shows half of the nebula and most of the Rosette cloud. The massive stars powering the nebula lie to the right of the image but are invisible at these wavelengths. Each colour represents a different temperature of dust, from -263ºC (only 10ºC above absolute zero) in the red emission to -233ºC in the blue.
The bright smudges are dusty cocoons hiding massive protostars. These will eventually become stars containing around ten times the mass of the Sun. The small spots near the centre and in the redder regions of the image are lower mass protostars, similar in mass to the Sun.
ESA's Herschel space observatory collects the infrared light given out by dust. This image is a combination of three infrared wavelengths, colour-coded blue, green and red in the image, though in reality the wavelengths are invisible to our eyes. It was created using observations from Herschel's Photoconductor Array Camera and Spectrometer (PACS) and the Spectral and Photometric Imaging Receiver (SPIRE).
Herschel is showing astronomers such young, massive protostars for the first time, as part of the 'Herschel imaging survey of OB Young Stellar objects'. Known as HOBYS, the survey targets young OB class stars, which will become the hottest and brightest stars.
"High-mass star-forming regions are rare and further away than low-mass ones," says Frédérique Motte, Laboratoire AIM Paris-Saclay, France. So astronomers have had to wait for a space telescope like Herschel to reveal them.
It is important to understand the formation of high-mass stars in our Galaxy because they feed so much light and other forms of energy into their parent cloud they can often trigger the formation of the next generation of stars.
When astronomers look at distant galaxies, the star-forming regions they see are the bright, massive ones. Thus, if they want to compare our Galaxy to distant ones they must first understand high-mass star-formation here.
"Herschel will look at many other high-mass star-forming regions, some of them building stars up to a hundred times the mass of the Sun," says Dr Motte, who plans to present the first scientific results from HOBYS at ESA's annual ESLAB symposium to be held in the Netherlands, 4-7 May.
Biggest Comet Measured to Date: Comet McNaught
Analysis of magnetometer data shows evidence of a shockwave surrounding the comet created when ionized gas emitted from the comet's nucleus interacts with fast-flowing particles in the solar wind, causing the wind to slow down abruptly.
In January and February 2007, Comet C/2006 P1 McNaught became the brightest comet visible from Earth for 40 years. Serendipitously, Ulysses made an unexpected crossing of Comet McNaught's tail during this time, one of three unplanned encounters with comet tails during the 19-year mission. The other encounters included Comet Hyakutake in 1996, the current record-holder for the comet with the longest tail.
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