HAT-P-1b

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**HAT-P-1b**
Parent star
Extrasolar planet		Lists of extrasolar planets
Star		ADS 16402 B
Constellation		Lacerta
Right ascension	(α)	22^h 57^m 47^s
Declination	(δ)	+38° 40′ 30″
Spectral type		G0V
Orbital elements
Semimajor axis	(a)	0.0551 ± 0.0015 AU
Eccentricity	(e)	0 (assumed)
Orbital period	(P)	4.46529 ± 0.00009 d
Inclination	(i)	85.9 ± 0.8°
Longitude of periastron	(ω)	?°
Time of periastron	(τ)	2,453,984.397 ± 0.009 JD
Physical characteristics
Mass	(m)	0.59 ± 0.04 M_J
Radius	(r)	1.36^+0.11_−0.09 R_J
Density	(ρ)	290 ± 30 kg/m³
Temperature	(T)	? K
Discovery information
Discovery date		2006
Discoverer(s)		HATNet Project
Detection method		Transit, radial velocity
Discovery status		Confirmed

HAT-P-1b is an extrasolar planet orbiting the Sunlike star ADS 16402 B. ADS 16402 B is the dimmer component of the ADS 16402 binary star system. It is located 450 light years away from Earth in the constellation Lacerta. As of 2006, HAT-P-1b has the largest radius and lowest density of any known extrasolar planet.

1 Discovery
2 Orbit and mass
3 Characteristics
4 References

[edit] Discovery

HAT-P-1b was detected by searching for astronomical transits of the parent star by orbiting planets. As the planet passes in front of its parent star (as seen from Earth), it blocks a small amount of the light reaching us from the star. HAT-P-1b was first detected by a dip of 0.6% in the light from the star. This enabled determination of the planet's radius and orbital period. The discovery was made by the HATNet Project (Hungarian Automated Telescope Network) using telescopes in Arizona and Hawaii and announced on September 14, 2006.^[1]

[edit] Orbit and mass

HAT-P-1b is located in a very close orbit to its star, taking only 4.46 days to complete.^[2] It therefore falls into the category of hot Jupiters. At only 8.24 million kilometers from the star, tidal forces would circularise the orbit unless another perturbing body exists in the system. At the present time, the existing measurements are not sufficient to determine the orbital eccentricity, so a perfectly circular orbit has been assumed by the discoverers.^[3]

In order to determine the mass of the planet, measurements of the star's radial velocity variations were made by the N2K Consortium. This was done by observing the Doppler shift in the star's spectrum. Combined with the known inclination of the orbit as determined by the transit observations, this revealed the mass of the planet to be 0.53 times that of Jupiter.^[2]

[edit] Characteristics

As evidenced by its high mass and planetary radius, HAT-P-1b is a gas giant, most likely composed primarily of hydrogen and helium. The planet would thus have no well-defined surface. Current theories predict that such planets formed in the outer regions of their solar systems and migrated inwards to their present orbits.

HAT-P-1b currently holds a record for being the least dense extrasolar planet discovered, and is significantly larger than predicted by theoretical models.^[1] This may indicate the presence of an additional source of heat within the planet. One possible candidate is tidal heating from an eccentric orbit, a possibility which has not been ruled out from the available measurements. However, another planet with a significantly inflated radius, HD 209458 b, is in a circular orbit.

An alternative possibility is that the planet has a high axial tilt, like Uranus in our solar system. The problem with this explanation is that it is thought to be quite difficult to get a planet into this configuration, so having two such planets among the set of known transiting planets is problematic.