by Paul Gilster | May 4, 2007 | Exoplanetary Science |
The early news from COROT couldn’t be more encouraging. Just sixty days into its science mission, the spacecraft has found its first transiting exoplanet and has returned information about the interior of a star. The latter findings point to COROT’s role in asteroseismology, the study of stellar interiors through analysis of the star’s light curves. What has mission scientists smiling is that in both cases, the instruments they’re watching show signs of working even better than planned.
True, the data carrying these results are still noisy and in need of plenty of analysis, but COROT project scientist Malcolm Fridlund sounds quite an optimistic note:
“The data we are presenting today is still raw but exceptional. It shows that the on-board systems are working better than expected in some cases – up to ten times the expectation before launch. This will have an enormous impact on the results of the mission.”
One would think so. It raises the stakes for COROT from detecting ‘super-Earths’ to finding an Earth-sized planet in transit. For now, though, we can be content with COROT-Exo-1b, a hot gas giant with radius 1.78 times that of Jupiter. Its primary is a yellow dwarf not dissimilar from the Sun in the direction of the constellation Monoceros, some 1500 light years away. Followup spectroscopic studies from the ground have determined a mass of 1.3 times Jupiter’s. The planet’s orbital period is about 1.5 days.
Image: This image shows the signature of the presence of a planet orbiting a star. The intensity of light coming from the star is represented on the y-axis whereas the x-axis shows the phase, or the revolution of the planet around the star. The amount of light from the star reaching COROT decreases each time the planet passes in front of the star itself. This is when the drop is registered. This was the first planet detected by COROT since the beginning of its mission. This light curve is part of a data set obtained between February and April 2007. Credit: COROT exo-team.
As to asteroseismology, COROT’s 50-day study of a Sun-like star (not the one that is home to the new exoplanet) showed luminosity variations on short time scales (a few days), perhaps related to the star’s magnetic activity. It will be fascinating to watch the science of ‘starquakes’ and other stellar events mature. COROT’s findings will provide data that should be useful in the challenging task we discussed the other day, determining the age of a given star.
This morning I asked Dr. Fridlund (via e-mail) how COROT separated its exoplanetary work from its asteroseismology, or whether the two were engaged in simultaneously on the same star. His response:
COROT has two cameras in the focal plane. One for asteroseismology and one for exo-planetology. They have different characteristics, since the stars you study for asteroseismological variations are brighter. In the former you have maybe a few hundred targets of different types (of which the published one is one of the brightest of the ones resembling our Sun). In the exoplanet field, on the other hand you have thousands of targets. In this first field about 6,000, and in the one planned with the most objects (the next one starting on 16 May and running until 16 October) there are no less than 12,000.
Let’s hope the good news continues (and congratulations to the COROT team for an outstanding mission thus far). While we await the upcoming Kepler mission and listen to the continuing debate over what technologies a Terrestrial Planet Finder should deploy to get actual images of distant worlds, COROT is already hard at work, the first space mission flown with the explicit purpose of detecting extrasolar planets. A French project developed with the help of the European Space Agency, COROT keeps Europe’s planet-hunting initiatives in the spotlight.
by Paul Gilster | May 3, 2007 | Exoplanetary Science |
Last night I was thinking that the day would come when all the planets we’ve been discovering have proper names instead of stark designations in catalogs. Then I realized that this is unlikely. As the rate of planetary discoveries accelerates through space-borne missions and ever more precise detection methods here on Earth, it may be that we’ll keep generating new finds faster than the naming process can catch up with them. So I guess we should get used to designations like Gliese 581 c.
Of course, a planet can have multiple designations, depending on how it’s catalogued or found. The recently announced gas giant HAT-P-2b (a very strange place indeed) is called this not for its place in a catalog but its discovery method, the HAT network of automated telescopes. HAT stands for Hungarian-made Automated Telescope, but the project is headquartered at the Harvard-Smithsonian Center for Astrophysics (CfA) and works with instruments in Arizona, Hawaii and in this case, Israel. Its focus: The detection of transiting exoplanets like this one.
But HAT-P-2b is also HD 147506 b, referring to its star’s listing in the Henry Draper Catalog. You can see that designations like these lack a certainty poetry, depending on the catalog or project involved (think of TrES-1, designating the Trans-Atlantic Exoplanet Survey that found it — examples begin to multiply). We’ll have to draw our aesthetic satisfaction not from colorful names but the exotic places these designations conjure up. And HAT-P-2b by any name turns out to be exotic indeed. Gaspar Bakos (CfA) calls it “…a really weird planet.”
The parent F-class star, HD 147506, lies 440 light years away in the constellation Hercules. Larger and brighter than the Sun, it’s home to at least one planet and perhaps another. That thinking is based on the odd elliptical orbit of HAT-P-2b, which represents a marked break from the circular orbits thus far found with transiting gas giants, all of which have been ‘hot Jupiters.’ With an orbital period of a scant 5.63 days, HAT-P-2b comes into transit every five days and 15 hours. Its oval orbit moves it from periastron at 3.1 million miles to fully 9.6 million miles at apastron, an eccentricity that is probably the result of an outer planet, although at this juncture there is no hard evidence of that planet’s existence.
This one is no hot Jupiter; in fact, it’s much closer to a failed star. The co-author on the discovery paper, Dimitar Sasselov, says this: “With 50 percent more mass, it could have begun nuclear fusion for a short time.” Even so, HAT-P-2b is one dense world, eight times the density of Jupiter, but evidently packed into a ball only slightly larger than that planet. It stands out compared to previously identified transiting exoplanets.
From the discovery paper:
Its mass of 8.17 ± 0.72MJ is ~5 times greater than any of these 14 other exoplanets. Its mean density ? = 6.6 ± 2.7 g cm?3 is ~5 times that of the densest known exoplanet (OGLE-TR-113b, ? = 1.35 g cm?3 ) and indeed greater than that of Earth (? = 5.5 g cm?3) or other solar system rocky planets. Its surface gravity of 149 ± 13 m s?2 is 5 times that of any of the previously known TEPs, and 20 times that of HAT-P-1b.
A mean density greater than Earth’s, and notice that last sentence, which points out the surface gravity is five times that of any transiting exoplanets (TEPs) previously found. Expect no media buzz with this one. A massive gas giant in a peculiar orbit around a hot F-class star doesn’t offer much by way of habitability potential. But most planets aren’t going to be habitable, and every transit we find offers the chance to measure the planet’s physical size from the degree to which the star’s light is dimmed by its passage. The paper is Bakos et al., “HAT-P-2b: A Super-Massive Planet in an Eccentric Orbit Transiting a Bright Star,” submitted to the Astrophysical Journal, with abstract available.
by Paul Gilster | May 2, 2007 | Exoplanetary Science |
We need to know more about how stars age. Ponder this: Centauri A and B are perhaps 2.5 billion years older than our Sun. If we’re interested in the development of intelligent life, older is clearly better — who knows what Earth might develop in the next two billion years? But are there planets around either of the primary Centauri stars? And if there are, how have their planetary systems changed over the course of those milennia?
Addendum: See the comments below — my figure of 2.5 billion years older than the Sun is in the middle of more extreme age estimates in both directions, and even these are questioned by the work we discuss in the following paragraphs.
One way to study these things is by looking at how stars rotate. A recently announced method called gyrochronology works with the premise that a star’s age is tightly bound up with both its rotation and its color. Syndey Barnes, who developed the technique at Lowell Observatory, explains it this way:
“If you know the relationship between three quantities, measuring two of them allows you to calculate the third. The relationship between age, color, and rotation period has particular and useful mathematical properties that simplify the analysis and allow the uncertainties to be calculated easily.”
If Barnes is right, we have a way to calculate a stellar age within about 15 percent. That compares well with existing techniques, where the uncertainties can range from 50 to 100 percent, and offers us the ability to calculate ages for individual solar-type stars using only their rotation periods and colors. By contrast, the so-called isochrone method in use today works better in star clusters than with individual stars. Moreover, unlike gyrochronology, its computations of evolutionary trajectories for stars are less accurate for stars on the main sequence, those considered most intriguing as we look for possible exoplanets supporting life.
Another established method for calculating a star’s age is to measure emissions from its chromosphere, but the uncertainty here is much higher than with gyrochronology. Given that the Kepler mission will be measuring the rotation periods of thousands of stars as it looks for planetary transits, the gyrochronology method can become a useful adjunct in our exoplanetary studies, helping us weigh the planetary systems we find against their probable age and evolution.
Barnes’ work is unlikely to replace earlier methods. But working in tandem with isochronic and chromospheric techniques, gyrochronology could help us broaden our knowledge of stellar ages across a wider range of stellar types both on and off the main sequence. Barnes ends his paper on an upbeat note: “Thus, we have re-investigated the use of a rotating star as a clock, clarified and improved its usage, calibrated it using the Sun, and demonstrated that it keeps time well.”
Addendum: Note what a sharp-eyed reader saw in Barnes’ paper about the Centauri stars, which I had missed: “…we derive ages for the components [Centauri A and B] of 3.93 Gyr and 3.84 Gyr, with a mean of 3.9±0.6 Gyr, toward the lower end of the published ages, but in good agreement with one another.” That’s quite a difference from chromospheric values of 5.62 and 4.24 Gyr, and isochronic values of 7.84 Gyr and >11.36 Gyr! We’ve got work to do to reconcile such numbers.
The paper is Barnes, “Ages for illustrative field stars using gyrochronology: viability, limitations and errors,” accepted for publication in The Astrophysical Journal, abstract and paper available here.
by Paul Gilster | May 1, 2007 | Culture and Society
Bohdan Paczynski, the Princeton astrophysicist who died April 19, was a major contributor to the exoplanet hunt, fine-tuning the techniques used in gravitational microlensing. Coming to Princeton in 1982 after twenty years at Warsaw’s Copernicus Astronomical Center, Paczynski understood early on that the bending of light by foreground objects, predicted by Einstein, could be applied to surveying stars in our own galaxy. A star passing directly in front of another can focus the light from the background star, producing a natural lens that allows investigation of objects otherwise impossible to observe.
The consortium of scientists that Paczynski led would go on to found the Optical Gravitational Lensing Experiment (OGLE), now active at Chile’s Las Campanas Observatory. Says Michael Strauss, a professor of astrophysical sciences and a colleague of Paczynski’s:
“The next thing Bohdan realized was that when you observe lensing that often, you can do other clever things, such as use it to look for planets around faraway stars. If there’s a planet orbiting the foreground star, the planet will subtly change the lensing signature, which can be used to infer the existence of the planet.”
And that’s just what OGLE did last year in discovering one of the smallest planets yet found, a rocky world orbiting a star some 20,000 light years away. Gravitational lensing provides a unique window into the distant stellar system and helps us expand our exoplanetary census as we look toward galactic center. With solid contributions in dark energy and gamma ray bursts, Paczynski’s contributions were significant, his life cut off all too soon. The astrophysicist died of brain cancer at the age of 67. This Princeton news release summarizes a remarkable career.
by Paul Gilster | May 1, 2007 | Culture and Society
Having had time to decompress from their exhausting Asian trip, Gregory and Elisabeth Benford have revised and enlarged the account of their travels that Centauri Dreams published in late March. The text contains numerous additional insights, but what makes this revision truly stand out from the original post are the photographs, fully seven times the number first published, each illustrating a unique facet of their journey. I’ve inserted the new Asia Emerging in the archives. Don’t miss its unforgettable images and insights into a part of the world that will have much to say about our future on this planet. I told Gregory that my favorite photograph is the one of him at the Raffles Hotel in Singapore, Singapore Sling in hand. Shades of Somerset Maugham…
