HD 149026 b

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HD 149026 b

Orbital elements
Semi-major axis a:	0.042 AU
Eccentricity e:	(fixed) 0
Orbital period P:	2.8766 ± 0.001 d
Inclination i:	85.3°
Longitude of periastron ω:	-°
Time of periastron τ:	2,453,530.751 (transit time) JD
Physical characteristics
Mass:	0.36 ± 0.03 MJ
Radius:	0.725 ± 0.03 RJ
Density:	1170 kg/m³
Temperature:	1540 K
Discovery
Discovery date:	2005
Detection method(s):	radial velocity, transit
Discoverer(s):	B. Sato, D. Fischer, G. Henry et al.

HD 149026 b, is a transiting extrasolar planet orbiting the star HD 149026. It is notable for being exceptionally dense for a giant planet.

Contents 1 Discovery 2 Characteristics 3 Theoretical consequences 4 External links 5 References

[edit] Discovery

The planet was discovered by the N2K Consortium which searches stars for closely orbiting giant planets similar to 51 Pegasi b using the highly successful radial velocity method. The spectrum of the star was studied from the Keck and Subaru Telescopes. After the planet was first detected from the Doppler effect it caused in the light of the host star, it was studied for transits at the Fairborn Observatory. A tiny decrease of light (0.003 magnitudes) was detected every time the planet was transiting the star, thus confirming its existence. ^[1]

Although the change of brightness caused by the transiting planet is tiny, it is detectable by amateur astronomers, providing an opportunity for amateurs to make important astronomical contributions. Indeed, one amateur astronomer, Ron Bissinger, actually detected a partial transit a day before the discovery was published. ^[2]

[edit] Characteristics

The planet orbits the star in a so-called "torch orbit". One revolution around the star takes only a little less than three Earth days to complete. The planet is less massive than Jupiter (0.36 times Jupiter's mass, or 114 times Earth's mass). Although the exact temperature of the planet is not known, it is estimated that the temperature is about 1540 K based on a reasonable albedo value of 0.3. Therefore the planet is a borderline case between the Class IV and Class V planets in the Sudarsky classification scheme. ^[1]

There are a number of such "hot Saturns", but HD 149026 b is so far unique. Most transiting planets have radii comparable to Jupiter's or more; however, the radius of HD 149026 b is only about 73% that of Jupiter, meaning that the planet is very dense for a gas giant. It may have an exceptionally large core composed of heavier elements. Theoretical models give the core a mass of 70 times Earth's mass. In fact, the planet may have more rocky and metallic material than all the planets in our Solar system combined. ^[1]

Because of the high density, gravity must be extreme on the surface of the core. It is calculated to be nearly 10 g, or 10 times the gravity on Earth's surface.

[edit] Theoretical consequences

The discovery was advocated as a piece of evidence for the popular solar nebula accretion model, where planets are formed from the accretion of smaller objects. In this model, giant planet embryos grow large enough to acquire large envelopes of hydrogen and helium. However, opponents of this model emphasize that only one example of such dense planet is not a proof. In fact, such a huge core is difficult to explain even by the core accretion model. ^[1]

One possibility is that because the planet orbits so close its star, it is — unlike Jupiter — ineffective in cleansing the planetary system of rocky bodies. Instead, a heavy rain of heavier elements on the planet may have helped creating the large core. ^[1]

Careful radial velocity measurements have made it possible to detect the Rossiter-McLaughlin effect, the shifting in photospheric spectral lines caused by the planet occulting a part of the rotating stellar surface. This effect allows the measurement of the angle between the planet's orbital plane and the equatorial plane of the star. In the case of HD 149026 b, the alignment was measured to be +11° ± 14°. This in turn suggests that the formation of the planet was peaceful and probably involved interactions with the protoplanetary disc. A much larger angle would have suggested a violent interplay with other protoplanets. ^[3]

[edit] External links

• Extrasolar Planets Encyclopaedia entry — URL accessed on May 7, 2006
• N2K Information For Star HD149026 from N2K Consortium — URL accessed on May 7, 2006
• One Big Ball of Rock — Sky & Telescope magazine — URL accessed on May 7, 2006
• Amateur Detects New Transiting Exoplanet — Sky & Telescope magazine — URL accessed on May 7, 2006

[edit] References