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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[1]
Right ascension (α) 22h 57m 46.844s[2]
Declination (δ) +38° 40′ 30.33″[2]
Distance 450+72
(139 +22
[3] pc)
Spectral type G0V[3]
Orbital elements
Semi-major axis (a) 0.05561 ± 0.00083[4] AU
Eccentricity (e) <0.067[5]
Orbital period (P) 4.4652968 ± 0.0000018[6] d
Inclination (i) 85.634 ± 0.056[4]°
Time of transit (Tt) 2,453,979.93165 ± 0.00025[6] JD
Semi-amplitude (K) 59.3 ± 1.4[5] m/s
Physical characteristics
Mass (m) 0.529 ± 0.020[6] MJ
Radius (r) 1.319 ± 0.019[4] RJ
Temperature (T) 1322 ± 15[4]
Discovery information
Discovery date Thurs, Sept 14, 2006[7]
Discoverer(s) HATNet Project[3]
Discovery method Transit, radial velocity[3]
Discovery site Arizona and Hawaii[3]
Discovery status Published[3]

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 roughly 450 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.[7]

Orbit and mass[edit]

HAT-P-1b is located in a very close orbit to its star, taking only 4.47 days to complete.[3] 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.[8] 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.[3]


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.[9] This is +3.6 ± 2.0 degrees.[5]


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.[7] 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.[10] 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. ^ Roman, Nancy G. (1987). "Identification of a Constellation From a Position". Publications of the Astronomical Society of the Pacific. 99 (617): 695–699. Bibcode:1987PASP...99..695R. doi:10.1086/132034.  Vizier query form
  2. ^ a b Zacharias, N.; et al. (2013). "The Fourth US Naval Observatory CCD Astrograph Catalog (UCAC4)". The Astronomical Journal. 145 (2). 44. Bibcode:2013AJ....145...44Z. arXiv:1212.6182Freely accessible. doi:10.1088/0004-6256/145/2/44.  Vizier catalog entry for ADS 16402B
  3. ^ a b c d e f g h 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. Bibcode:2007ApJ...656..552B. arXiv:astro-ph/0609369Freely accessible. doi:10.1086/509874. 
  4. ^ a b c d Nikolov, N.; et al. (2014). "Hubble Space Telescope hot Jupiter transmission spectral survey: a detection of Na and strong optical absorption in HAT-P-1b". Monthly Notices of the Royal Astronomical Society. 437 (1): 46–66. Bibcode:2014MNRAS.437...46N. arXiv:1310.0083Freely accessible. doi:10.1093/mnras/stt1859. 
  5. ^ a b c Johnson, John Asher; et al. (2008). "Measurement of the Spin-Orbit Angle of Exoplanet HAT-P-1b". The Astrophysical Journal. 686 (1): 649–657. Bibcode:2008ApJ...686..649J. arXiv:0806.1734Freely accessible. doi:10.1086/591078. 
  6. ^ a b c Turner, Jake D.; et al. (2016). "Ground-based near-UV observations of 15 transiting exoplanets: constraints on their atmospheres and no evidence for asymmetrical transits". Monthly Notices of the Royal Astronomical Society. 459 (1): 789–819. Bibcode:2016MNRAS.459..789T. arXiv:1603.02587Freely accessible. doi:10.1093/mnras/stw574. 
  7. ^ a b c Aguilar, David A.; Pulliam, Christine (September 14, 2006). "Strange New Planet Baffles Astronomers" (Press release). Cambridge, Massachusetts: Harvard–Smithsonian Center for Astrophysics. Retrieved November 18, 2016. 
  8. ^ "A HAT trick". Retrieved 2007-10-14. 
  9. ^ Winn, Joshua N. (2008). "Measuring accurate transit parameters". Proceedings of the International Astronomical Union. 4: 99. arXiv:0807.4929v2Freely accessible. doi:10.1017/S174392130802629X. 
  10. ^ Jackson, Brian; Richard Greenberg; Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". ApJ. 681 (2): 1631. Bibcode:2008ApJ...681.1631J. arXiv:0803.0026Freely accessible. 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″