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Planetary News: Extrasolar Planets (2005)

Extrasolar Planets:

Two Teams Claim Its Picture of Distant Planetary-Mass Object is First, but Neither is Official Yet

A team of French and American astronomers announced this past weekend it had confirmed that a picture of what it suggests is a giant extra solar planet (ESP) approximately five times the mass of Jupiter -- which it initially released last September -- is the first direct image of a planet around another star. That announcement came on the heels of another announcement earlier this month by a German team that claims it produced the first image of an ESP.

Acquiring the first actual picture of an ESP is an astronomically big deal and certainly will assure the team that secures that distinction a place in science history, so the fact that there is heated competition and dueling claims between groups is not at all surprising. But it may be a good long while before either group can officially own the bragging rights, according to Alan P. Boss, of the Carnegie Institution of Washington, chair of the ESP working group of the International Astronomical Union (IAU), the organization that has final authority on such matters.

Right now, there are issues with both objects being deemed planets, Boss said. "I bounced each of these discoveries off the IAU members and, in each case, roughly half are saying, 'No, this is not a planet,' and half are saying, 'We should call it a planet.' The compromise we came up with is to call them 'possible planetary-mass objects,' which probably doesn't satisfy anyone."

The object that the French-American team believes is a giant gas planet -- 2M1207b -- is orbiting a young brown dwarf at a distance of about 55 astronomical units (AU), nearly twice as far as Neptune is from our Sun. The 2M1207 system is more than 200 light years away, and is near the southern constellation of Hydra. The team -- led by astronomer Gael Chauvin, of the European Southern Observatory (ESO) -- first discovered 2M1207b last April, but there was some doubt about whether the star and planet were actually gravitationally bound or, in other words, part of the same system. In August, the team made additional observations with the Hubble Space Telescope (HST), and although those correlated with the initial observations, they were collected too soon after the discovery to conclusively demonstrate that the faint source was a planet.

Then, in September, the team released the image it took with the state-of-the-art NAOS-CONICA (NACO) adaptive optics camera mounted on the Very Large Telescope at the ESO in northern Chile. Still, it was impossible to prove that the faint source was not a background object, such as an unusual galaxy or a peculiar cool star with abnormal infrared colors, even though this appeared very unlikely to the team members.

So, in February and March of this year, Chauvin and colleagues took new images of the young brown dwarf and what they believe is its giant planet companion, again with the NACO instrument on the VLT. The new observations convinced the team that the two objects are moving together and hence are gravitationally bound.

"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 team member Benjamin Zuckerman, a professor of physics and astronomy at the University of California Los Angeles (UCLA), and a member of NASA's Astrobiology Institute.

The problem with 2M1207b, according to Boss, is that it is orbiting around a brown dwarf. "That's the crux of the whole matter," he said in an interview with The Planetary Society yesterday. The IAU's definition of a planet is that the object be 13.6 Jupiter masses or less and be in orbit around a star or star remnant. "A brown dwarf -- depending on who you talk to -- is not considered a star, so the IAU is worried about this," he said.

"There is no question that 2M1207b is a fascinating and exciting object to see," Boss continued. "But since it's in orbit around a brown dwarf, there is the feeling that it most likely formed at the same time and in the same way the brown dwarf did and the way stars do -- in a one-stage gravitational collapse. The object's mass is estimated to be only one-fifth of that of the primary [brown dwarf], and, because binary stars are really closely matched in masses, it really looks to me like the 2M1207 system is a binary star system or a binary brown dwarf system [made up of] a brown dwarf and sub-brown dwarf. Nature, however, is not easy on us. We're learning that the star formation process can produce objects as low in mass as the most massive planets, so there's an overlap between the two formation processes, and that complicates things."

Chauvin and colleagues don't see 2M1207b's formation process as an issue, at least not at this juncture. "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 in an official press release. "Instead it must have formed the same way our Sun formed, by a one-step gravitational collapse of a cloud of gas and dust."

Based on research of our own gas giant planets, scientists believe there are two possibilities for how these bodies form -- core accretion, where a heavy element core of rock or ice is formed and then gravity pulls in the hydrogen and helium gas envelope around it, and disc instability, which Boss first proposed, where the planet develops as a result of a gravitational instability in a star's protoplanetary disk -- and astronomers and planetary scientists cannot really say for sure which one holds or if both are processes have given birth to gas giants.

In any event, there is no question that the 2M1207 system, as the Chauvin team acknowledges, does have some unusual properties. "This is the first time a planet's ever been seen orbiting around a brown dwarf," Zuckerman said. "It's also the only companion to a brown dwarf that's been discovered that's out as far out as this is, at 55 AU. I believe all the other brown dwarf companions -- where the brown dwarf is a primary of the system -- have been within 20 AU and this is out at 55, and that, I think, is unique. I know some people quibble about the fact that it's orbiting around a brown dwarf rather than a star, but what we care about is measuring the properties of objects with planetary masses over a range of ages and whether or not 2M1207b is orbiting around something of 25 Jupiter masses -- a brown dwarf -- or 80 Jupiter masses, say, a low-mass star, is just second order at most. It's just not the important thing."

Boss, clearly, disagrees. And that, he said, is not the only issue with 2M1207b. "There is the same uncertainty about what 2M1207b's true mass is. The best estimate is 5 Jupiter masses, but that is not a dynamically-based mass, but a mass based on theoretical models, which is very unlike the masses we have from all the planets [discovered by radial velocity] where we really know dynamically what the mass is to within a small factor of about 1.3."

Indeed, the Chauvin team arrived at the 5 Jupiter-mass size of 2M1207b from the infrared colors and the spectral data, and evolutionary model calculations. But the object's mass was also estimated by a different method of analysis, noted Zuckerman, which focuses on the strength of its gravitational field. That technique suggests that the mass might be even less than 5 Jupiters. But, Boss pointed out, "there's interpretation here that's never been proven."

Nevertheless, Chauvin, Zuckerman, and colleagues remain convinced they have imaged the first ESP. The spectrum of 2M1207b, they have reported, presents a strong signature of water molecules, which indicates that it must be cold, and is typical of a planet with an atmosphere and not of a star or brown dwarf.

The paper describing this team's research has been accepted for publication in the journal Astronomy and Astrophysics and will hit print soon, perhaps along with another paper the team submitted on 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 other 'first picture' produced and released earlier this month by the German team -- led by Ralph Neuhäeuser, director of the Astrophysical Institute and University Observatory in Jena, Germany -- is of an orb traveling around a star called GQ Lupi [also known as GQ Lupus and GQ Lup], a star system that is about 450 light years away. Interestingly, that image was also acquired with the NACO instrument on the VLT in Chile, and Astronomy and Astrophysics also accepted this team's research for publication. And, like 2M1207b, GQ Lupi B has its share of uncertainties.

Although the Neuhäeuser team has calculated GQ Lupi A's companion -- which is still in the process of contraction [formation] -- to be around 2.4 Jupiter masses, within the accepted range for planetary masses, some in the astronomy community believe that GQ Lupi B might be a brown dwarf. That's because Neuhäeuser and colleagues, like the Chauvin team, had to rely on models that predict a planet's size based on its age, luminosity, and spectrum, and those models can be quite inaccurate for such a young star system.

Another uncertainty, Boss said, is the age of GQ Lupi B. "We think the GQ Lupus system is roughly 1 million years old, which is so young that it brings with it another pathological uncertainty -- which is you really then have to understand how that planet formed, and we don't really know how gas giant planets form."

The Neuhäeuser team makes the assumption that the possible planetary-mass object it discovered formed by core accretion. "But we don't know if that's right," Boss said. "Those are all big question marks involved in trying to figure out what the true mass is; whereas compared with the radial velocity methods -- biff, bam, boom -- you have a Sin(i) inclination uncertainty, which is maybe a factor of 1.3, but that is tiny compared to these uncertainties we're talking about with this particular object."

Additionally, Boss added, "another uncertainty is that different people have different assumptions in their calculations and they make different predictions about how bright it should be at a given age depending on its mass. For the German team object -- there's a variation of a factor of like 20 or 30 or 40 -- that's huge. Those uncertainties are why GQ Lupus still has question marks around it."

Unfortunately, astronomers do not yet have any systems where they can compare the model masses with the true dynamical masses. "So until we have such a system, there will always be that question mark -- and that's true of the both the 2M1207 object as well as the GQ Lupus object -- they are both uncertain because of that," explained Boss.

"What we really need to find is a system where the planetary-mass object is in close enough to the star that we can actually see it orbit around the star and still be able to image it, or see the star wobble and get a dynamical mass, but also be able to see the planet directly to see what the luminosity is so we can then compare a prediction from a theoretical model of a luminosity with its true mass and calibrate those models that way," Boss elaborated. "We need a system where we can get both a dynamical mass, actually measure how rapidly things are orbiting around each other and use Kepler's laws to tell us how massive they have to be and then we'll be able to calibrate these other models and really know which ones are right."

Right now, the astronomy community is not anywhere near achieving that goal. "We're not close at all," confirmed Boss. "With regard to the GQ Lupus system, we'd have to follow it for 1000 years" [since the possible planetary-mass companion there takes around 1200 years to complete one orbit]. But when we do get to that system, we'll be able to go back to these other ones and say, 'Oh here's what it really is.'"

The final answer as to which one, if either, of these two images are the bona fide first picture of an ESP may not be known for another decade. "We may have to wait for something like Terrestrial Planet Finder -- due to launch in 2016 -- or Darwin to do this because they have the ability to image planets in very close to the host star," Boss noted, "But even they will not measure directly the mass of the planet -- that would have to be done by another NASA project, the Space Interferometry Mission, scheduled for launch in 2010."

In the meantime, whatever the outcome of this current 'competition,' team members know that these images herald the future. "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," said Anne-Marie Lagrange, a member of the French-American team from the Grenoble Observatory in France.