A planet circling the star HD 149026 is certainly not the most massive extrasolar world we’ve discovered. But it does take honors on one count: it boasts the largest solid core ever found. Detected by a consortium of American, Japanese and Chilean astronomers, the planet is roughly equal to Saturn in mass though significantly smaller in diameter. It is being studied not only by analyzing its gravitational effects on HD 149026, but also by virtue of the fact that it transits the face of the star, dimming the starlight and allowing much more extensive measurements of its size, mass and density.
Located some 250 light years from Earth, the planet takes 2.87 days to circle its primary. Modeling its structure provides indications that the new planet’s core is 70 times the mass of the Earth. And that gets us into interesting territory, for it has implications for our theories about how planets form. The so-called ‘core accretion’ theory of planet formation says that planets begin as small cores of rock and ice that gradually grow as they acquire new mass through their gravitational pull.
The other theory is ‘gravitational instability,’ which suggests that planets form as a dense cloud collapses. Some of the scientists studying HD 149026 see its huge core as evidence for the core accretion model. “Without observational evidence, either theory is viable,” said Fischer, assistant professor of astronomy at SFSU and leader of Next 2,000 Planets (N2K), a consortium of American, Japanese and Chilean astronomers formed last year to search for extra-solar planets. “But we believe that the large, rocky core of this planet couldn’t have formed by cloud collapse. Instead we think it must have grown a core first, then acquired gas.”
Image: An artist’s conception of the planet HD 149026b as it moves in front of its parent star. Credit: Lynette Cook.
How could a core of this size form? One theory is that two protoplanets with heavy cores may have collided, which would have produced an elongated orbit, but one which, over time, might have been brought into its present state by tidal forces. Follow-up observations may provide evidence for this by showing a tilt in the planet’s orbit with respect to the rotation axis of the star.
Centauri Dreams‘ take: It seems premature to say that this finding clinches one model of planetary formation over another. What it does show is that the core accretion model makes sense in this case, while not ruling out gravitational instability as a model for other planets. But isn’t it fascinating to watch as we move from pure detection of extrasolar worlds to actual analysis based on ever more detailed observations? This process will continue at a fast pace as we move closer toward the ultimate goal: the detection and analysis of Earth-like worlds around nearby stars.
The paper is Bun’ei Sato, Debra A. Fischer, Gregory W. Henry et al., “The N2K Consortium. II. A Transiting Hot Saturn Around HD 149026 With a Large Dense Core,” which will appear in the Astrophysical Journal, and is currently available online (PDF warning). A news release from San Francisco State University is available here.
Five New Transits of the Super-Neptune HD 149026
Authors: Joshua N. Winn, Gregory W. Henry, Guillermo Torres, Matthew J. Holman
(Submitted on 12 Nov 2007)
Abstract: We present new photometry of HD 149026 spanning five transits of its “super-Neptune” planet. In combination with previous data, we improve upon the determination of the planet-to-star radius ratio: R_p/R_star = 0.0491^{+0.0018}_{-0.0005}. We find the planetary radius to be 0.71 +/- 0.05 R_Jup, in accordance with previous theoretical models invoking a high metal abundance for the planet. The limiting error is the uncertainty in the stellar radius. Although we find agreement among four different ways of estimating the stellar radius, the uncertainty remains at 7%. We also present a refined transit ephemeris and a constraint on the orbital eccentricity and argument of pericenter, e cos(omega) = -0.0014 +/- 0.0012, based on the measured interval between primary and secondary transits.
Comments: To appear in ApJ [19 pages]
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0711.1888v1 [astro-ph]
Submission history
From: Joshua N. Winn [view email]
[v1] Mon, 12 Nov 2007 23:02:34 GMT (112kb)
http://arxiv.org/abs/0711.1888
Discovering the Growth Histories of Exoplanets: The Saturn Analog HD 149026b
Authors: Sarah E. Dodson-Robinson (1), Peter Bodenheimer (2) ((1) NASA Exoplanet Science Institute/Caltech, (2) UCO/Lick Observatory)
(Submitted on 6 Jan 2009)
Abstract: The transiting “hot Saturn” HD 149026b, which has the highest mean density of any confirmed planet in the Neptune-Jupiter mass range, has challenged theories of planet formation since its discovery in 2005. Previous investigations could not explain the origin of the planet’s 67 Earth-mass solid core without invoking catastrophes such as gas giant collisions or heavy planetesimal bombardment launched by neighboring planets.
Here we show that HD 149026b’s large core can be successfully explained by the standard core accretion theory of planet formation. The keys to our reconstruction of HD 149026b are (1) applying a model of the solar nebula to describe the protoplanet nursery; (2) placing the planet initially on a long-period orbit at Saturn’s heliocentric distance of 9.5 AU; and (3) adjusting the solid mass in the HD 149026 disk to twice that of the solar nebula in accordance with the star’s heavy element enrichment.
We show that the planet’s migration into its current orbit at 0.042 AU is consistent with our formation model. Our study of HD 149026b demonstrates that it is possible to discover the growth history of any planet with a well-defined core mass that orbits a solar-type star.
Comments: 11 pages, including 3 figures. Submitted to ApJ Letters
Subjects: Astrophysics (astro-ph)
Cite as: arXiv:0901.0582v1 [astro-ph]
Submission history
From: Sarah Dodson-Robinson [view email]
[v1] Tue, 6 Jan 2009 00:11:33 GMT (343kb)
http://arxiv.org/abs/0901.0582