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HAT-P-1 b
Exoplanet List of exoplanets
Exoplanet Comparison HAT-P-1 b.png
Size comparison of HAT-P-1b with Jupiter.
Parent star
Star HAT-P-1 (ADS 16402 B)
Constellation Lacerta
Right ascension (α) 22h 57m 47s
Declination (δ) +38° 40′ 30″
Distance 453 ± 65 ly
(139 ± 20 pc)
Spectral type G0V [1]
Orbital elements
Semi-major axis (a) 0.0553 ± 0.0014[2] AU
Eccentricity (e) <0.067[2]
Orbital period (P) 4.4652934 ± 0.0000093[2] d
    (107.16704 h)
Inclination (i) 86.28 ± 0.20[2]°
Time of transit (Tt) 2,454,363.94656 ± 0.00072[2] JD
Semi-amplitude (K) 59.3 ± 1.4[2] m/s
Physical characteristics
Mass (m) 0.524 ± 0.031[2] MJ
Radius (r) 1.225 ± 0.059 [2] RJ
Discovery information
Discovery date Thurs, Sept 14, 2006
Discoverer(s) HATNet Project
Discovery method Transit, radial velocity
Discovery site Arizona and Hawaii
Discovery status Published

HAT-P-1b is an extrasolar planet orbiting the Sun-like star HAT-P-1, also known as ADS 16402 B. HAT-P-1 is the dimmer component of the ADS 16402 binary star system. It is located 453 light years away from Earth in the constellation Lacerta. HAT-P-1b is among the least dense of any of the known extrasolar planets.


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.[3]

Orbit and mass[edit]

HAT-P-1b is located in a very close orbit to its star, taking only 4.47 days to complete.[1] It therefore falls into the category of hot Jupiters. At only 8.27 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.[4] However, the eccentricity of the planet was calculated to be no greater than 0.067.

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±0.04 times that of Jupiter.[1]


As of August 2008, the most recent calculation of HAT-P-1b's Rossiter–McLaughlin effect and so spin-orbit angle was that of Johnson.[5] This is +3.6 ± 2.0 degrees.[2]


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 is significantly larger than predicted by theoretical models.[3] 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.[6] 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 the 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.


  1. ^ a b c Bakos, G. Á.; et al. (2007). "HAT-P-1b: A Large-Radius, Low-Density Exoplanet Transiting One Member of a Stellar Binary". The Astrophysical Journal. 656 (1): 552–559. arXiv:astro-ph/0609369free to read. Bibcode:2007ApJ...656..552B. doi:10.1086/509874. 
  2. ^ a b c d e f g h i Johnson, John Asher; et al. (2008). "Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b". The Astrophysical Journal. 686 (1): 649–657. arXiv:0806.1734free to read. Bibcode:2008ApJ...686..649J. doi:10.1086/591078. 
  3. ^ a b "Oversize Orb: Puffy planet poses puzzle". Science News Online. Archived from the original on 2007-10-13. Retrieved 2007-10-14. 
  4. ^ "A HAT trick". Retrieved 2007-10-14. 
  5. ^ Winn, Joshua N. (2008). "Measuring accurate transit parameters". Proceedings of the International Astronomical Union. 4: 99. arXiv:0807.4929v2free to read. doi:10.1017/S174392130802629X. 
  6. ^ Jackson, Brian; Richard Greenberg; Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ. 681 (2): 1631. arXiv:0803.0026free to read. Bibcode:2008ApJ...681.1631J. doi:10.1086/587641. 

External links[edit]

Media related to HAT-P-1b at Wikimedia Commons

Coordinates: Sky map 22h 57m 47s, +38° 40′ 30″