HD 149026 b

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HD 149026 b
Exoplanet Comparison HD 149026 b.png
Size comparison of HD 149026 b with Neptune and Jupiter.
Discovered byB. Sato,
D. Fischer,
G. Henry et al.[1]
Discovery siteW. M. Keck Observatory
Discovery date1 July 2005
Radial velocity
Orbital characteristics
0.042 AU (6,300,000 km)
2.87588874 ± 5.9×10−7[2] d
Semi-amplitude43.2 ± 2.6
StarHD 149026
Physical characteristics
Mean radius
0.725 ± 0.03 RJ
Mass0.36 ± 0.03 MJ
Mean density
1,252 kg/m3 (2,110 lb/cu yd)
Temperature2,300 K (2,030 °C; 3,680 °F)

HD 149026 b, formally named Smertrios /ˈsmɜːrtriɒs/, is an extrasolar planet approximately 250 light-years from the Sun in the constellation of Hercules.

The 2.8766-day period planet orbits the yellow subgiant star HD 149026 at a distance of 0.042 AU and is notable first as a transiting planet, and second for a small measured radius (relative to mass and incoming heat) that suggests an exceptionally large planetary core.


Following its discovery in 2005 the planet was designated HD 149026 b. In July 2014 the International Astronomical Union launched a process for giving proper names to certain exoplanets and their host stars.[3] The process involved public nomination and voting for the new names.[4] In December 2015, the IAU announced the winning name was Smertrios for this planet.[5] The winning name was submitted by the Club d'Astronomie de Toussaint of France. Smertrios was a Gallic deity of war.[6]


The planet was discovered by the N2K Consortium in 2005, 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.[7]


The planet's orbit is probably circular (within one standard deviation of error).[8]

Physical characteristics[edit]

Size comparison of HD 149026 b with Jupiter and Neptune.

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) but more massive than Saturn. The temperature of the planet was initially estimated on the basis of 0.3 Bond albedo to be about 1540 K,[1] above the predicted temperature of HD 209458 b (1400K), which had inaugurated the category of Chthonian "hell planet".[9] Its day-side brightness temperature was subsequently directly measured as 2,300 ± 200 K by comparing the combined emissions of star and planet at 8 μm wavelength before and during a transit event. This is around the boiling point of silicon and well above the melting point of iron.

This planet's albedo has not been measured directly. The initial estimate of 0.3 had come from averaging Sudarsky's theoretical classes IV and V. The planet's extremely high temperature has forced astronomers to abandon that estimate; now, they predict that the planet must absorb essentially all of the starlight that falls on it — that is, effectively zero albedo like HD 209458 b.[10] Much of the absorption takes place at the top of its atmosphere.

Between that and the hot, high-pressure gas surrounding the core, a stratosphere of cooler gas was once predicted[11] but has not been observed. The atmosphere is likely high in carbon monoxide and dioxide.[8]

The outer shell of dark, opaque, hot clouds are usually thought to be vanadium and titanium oxides ("pM planets"), but other compounds like tholins cannot be ruled out as yet.

The planet-star radius ratio is 0.05158 +/- 0.00077.[12] Currently what limits more precision on HD 149026 b's radius "is the uncertainty in the stellar radius",[13] and measurement of the stellar radius is distorted by pollution on the star's surface.[14]

Even allowing for uncertainty the radius of HD 149026 b is only about three quarters that of Jupiter (or 83% that of Saturn). HD 149026 b was the first of its kind:[15] HD 149026 b's low volume means that the planet is too dense for a Saturn-like gas giant of its mass and temperature.

It may have an exceptionally large core composed of elements heavier than hydrogen and helium:[1] the initial theoretical models gave the core a mass of 70 times Earth's mass; further refinements suggest 80-110 Earth masses.[16] As a result, the planet has been described as a "super-Neptune", in analogy to the core-dominated outer ice giants of our solar system, though whether the core of HD 149026 b is mainly icy or rocky is not currently known.[13] Robert Naeye in Sky & Telescope claimed "it contains as much or more heavy elements (elements heavier than hydrogen and helium) than all the planets and asteroids in our solar system combined".[17] In addition to uncertainties of radius, its tidal heating over its history needs be taken into account; if its current orbit is circular and if that had evolved from a more eccentric one, the extra heat increases its expected radius per its model and thereby its core radius.[18]

Naeye further speculated that the gravity could be as high as ten g (ten times gravity on Earth's surface) on the surface of the core.[17]

Theoretical consequences[edit]

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 a dense planet is not 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 to 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 create 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.[19] As of August 2008 this is still the canonical measurement.[20]

See also[edit]


  1. ^ a b c d e f Sato, Bun'ei; et al. (2005). "The N2K Consortium. II. A Transiting Hot Saturn around HD 149026 with a Large Dense Core". The Astrophysical Journal. 633 (1): 465–473. arXiv:astro-ph/0507009. Bibcode:2005ApJ...633..465S. doi:10.1086/449306.
  2. ^ Zhang, Michael; et al. (2018). "Phase Curves of WASP-33b and HD 149026b and a New Correlation between Phase Curve Offset and Irradiation Temperature". The Astronomical Journal. 155 (2). 83. arXiv:1710.07642. Bibcode:2018AJ....155...83Z. doi:10.3847/1538-3881/aaa458.
  3. ^ NameExoWorlds: An IAU Worldwide Contest to Name Exoplanets and their Host Stars. IAU.org. 9 July 2014
  4. ^ NameExoWorlds The Process
  5. ^ Final Results of NameExoWorlds Public Vote Released, International Astronomical Union, 15 December 2015.
  6. ^ NameExoWorlds The Approved Names
  7. ^ Naeye, Robert (July 7, 2005). "Amateur Detects New Transiting Exoplanet". Sky & Telescope.
  8. ^ a b Kevin B. Stevenson; et al. (2012). "Transit and Eclipse Analyses of Exoplanet HD 149026b Using BLISS Mapping". The Astrophysical Journal. 754 (2): 136. arXiv:1108.2057. Bibcode:2012ApJ...754..136S. doi:10.1088/0004-637X/754/2/136.
  9. ^ Hell planet gets solar hammering
  10. ^ Spaceflight Now | Breaking News | Exotic extrasolar planet is the hottest yet discovered
  11. ^ Ivan Hubeny; Adam Burrows (2008). "Spectrum and atmosphere models of irradiated transiting extrasolar giant planets". Proceedings of the International Astronomical Union. 4: 239–245. arXiv:0807.3588. Bibcode:2009IAUS..253..239H. doi:10.1017/S1743921308026458.
  12. ^ Nutzman, Philip; et al. (2008). "A Precise Estimate of the Radius of HD 149026b". Proceedings of the International Astronomical Union. 4: 466–469. arXiv:0807.1318. Bibcode:2009IAUS..253..466N. doi:10.1017/S1743921308026951.
  13. ^ a b Joshua N. Winn; et al. (March 2008). "Five New Transits of the Super-Neptune HD 149026b" (PDF). The Astrophysical Journal. 675 (2): 1531–1537. arXiv:0711.1888. Bibcode:2008ApJ...675.1531W. doi:10.1086/527032.
  14. ^ S.-L. Li; D. N. C. Lin; X.-W. Liu (2008). "Extent of pollution in planet-bearing stars". The Astrophysical Journal. 685 (2): 1210–1219. arXiv:0802.2359. Bibcode:2008ApJ...685.1210L. doi:10.1086/591122.
  15. ^ Since then, there is now KOI-196 b, a slightly larger "non-inflated hot Jupiter.
  16. ^ Burrows; Laughlin, Gregory; Henry, Gregory W.; Fischer, Debra A.; Marcy, Geoff; Butler, Paul; Vogt, Steve (2007). "Possible Solutions to the Radius Anomalies of Transiting Giant Planets". The Astrophysical Journal. 667 (1): 549–556. Bibcode:2007ApJ...667..549W. doi:10.1086/503354.
  17. ^ a b One Big Ball of Rock Robert Naeye, Sky & Telescope, last accessed October 13, 2007
  18. ^ Brian Jackson; Richard Greenberg; Rory Barnes (2008). "Tidal Heating of Extra-Solar Planets". The Astrophysical Journal. 681 (2): 1631–1638. arXiv:0803.0026. Bibcode:2008ApJ...681.1631J. doi:10.1086/587641.
  19. ^ Wolf; Laughlin, Gregory; Henry, Gregory W.; Fischer, Debra A.; Marcy, Geoff; Butler, Paul; Vogt, Steve (2007). "A Determination of the Spin-Orbit Alignment of the Anomalously Dense Planet Orbiting HD 149026". The Astrophysical Journal. 667 (1): 549–556. Bibcode:2007ApJ...667..549W. CiteSeerX doi:10.1086/503354.
  20. ^ Joshua N. Winn (2008). "Measuring accurate transit parameters". Proceedings of the International Astronomical Union. 4: 99–109. arXiv:0807.4929. Bibcode:2009IAUS..253...99W. doi:10.1017/S174392130802629X.

External links[edit]

Media related to HD 149026 b at Wikimedia Commons

Coordinates: Sky map 16h 30m 29.619s, +38° 20′ 50.31″