HD 149026 b
Encyclopedia
HD 149026 b is an extrasolar planet
approximately 257 light-year
s away in the constellation
of Hercules
. The planet was discovered after it transited its parent star, HD 149026
. It is notable for the presence of an exceptionally large planetary core suggested by measurements of its radius and mass.
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.
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.
The planet orbits the star in a so-called "torch orbit". One revolution around the star takes only a little less than three Earth day
s 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
, above the predicted temperature of HD 209458 b
(1400K) which had inaugurated the category of Chthonian
"hell planet". 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
. 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 is predicted. 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. Currently what limits more precision on HD 149026 b's radius "is the uncertainty in the stellar radius", and measurement of the stellar radius is distorted by pollution on the star's surface.
Even allowing for uncertainty the radius of HD 149026 b is only about three quarters that of Jupiter (or 83% that of Saturn). Only Gliese 436 b
of 52 planets known to transit their stars as of August 2008 is smaller in radius than HD 149026 b.
There are a number of such "hot Saturns
", but HD 149026 b is so far unique: HD 149026 b's low volume means that the planet is unexpectedly dense for a gas giant
of its mass and temperature. It may have an exceptionally large core composed of elements heavier than hydrogen and helium: the initial theoretical models gave the core a mass of 70 times Earth's mass; further refinements suggest 80-110 Earth masses. 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. 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". In addition to uncertainties of radius, its tidal heating over its history needs be taken into account; if its current orbit had evolved from a more eccentric one, the extra heat increases its expected radius per its model and thereby its core radius.
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.
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.
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 creating the large core.
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. As of August 2008 this is still the canonical measurement.
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
approximately 257 light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s away in the constellation
Constellation
In modern astronomy, a constellation is an internationally defined area of the celestial sphere. These areas are grouped around asterisms, patterns formed by prominent stars within apparent proximity to one another on Earth's night sky....
of Hercules
Hercules (constellation)
Hercules is a constellation named after Hercules, the Roman mythological hero adapted from the Greek hero Heracles. Hercules was one of the 48 constellations listed by the 2nd century astronomer Ptolemy, and it remains one of the 88 modern constellations today...
. The planet was discovered after it transited its parent star, HD 149026
HD 149026
HD 149026 is a yellow subgiant star approximately 257 light-years away in the constellation of Hercules. The star is thought to be much more massive, larger, and brighter than the Sun. , an extrasolar planet has been confirmed to be orbiting the star...
. It is notable for the presence of an exceptionally large planetary core suggested by measurements of its radius and mass.
Discovery
The planet was discovered by the N2K ConsortiumN2K Consortium
The N2K Consortium is a collaborative multinational effort by American, Chilean and Japanese astronomers to find additional extrasolar planets around stars that are...
in 2005, which searches stars for closely orbiting giant planets similar to 51 Pegasi b
51 Pegasi b
51 Pegasi b , sometimes though unofficially named Bellerophon, is an extrasolar planet approximately 50 light-years away in the constellation of Pegasus...
using the highly successful radial velocity
Doppler spectroscopy
Doppler spectroscopy, also known as radial velocity measurement, is a spectroscopic method for finding extrasolar planets. It involves the observation of Doppler shifts in the spectrum of the star around which the planet orbits....
method. The spectrum
Spectrum
A spectrum is a condition that is not limited to a specific set of values but can vary infinitely within a continuum. The word saw its first scientific use within the field of optics to describe the rainbow of colors in visible light when separated using a prism; it has since been applied by...
of the star was studied from the Keck and Subaru Telescopes
Subaru (telescope)
Subaru Telescope is the 8.2 metre flagship telescope of the National Astronomical Observatory of Japan, located at the Mauna Kea Observatory on Hawaii. It is named after the open star cluster known in English as the Pleiades...
. After the planet was first detected from the Doppler effect
Doppler effect
The Doppler effect , named after Austrian physicist Christian Doppler who proposed it in 1842 in Prague, is the change in frequency of a wave for an observer moving relative to the source of the wave. It is commonly heard when a vehicle sounding a siren or horn approaches, passes, and recedes from...
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.
Although the change of brightness caused by the transiting planet is tiny, it is detectable by amateur astronomers
Amateur astronomy
Amateur astronomy, also called backyard astronomy and stargazing, is a hobby whose participants enjoy watching the night sky , and the plethora of objects found in it, mainly with portable telescopes and binoculars...
, 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.
Physical characteristics
Day
A day is a unit of time, commonly defined as an interval equal to 24 hours. It also can mean that portion of the full day during which a location is illuminated by the light of the sun...
s to complete. The planet is less massive than Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
(0.36 times Jupiter's mass, or 114 times Earth's mass) but more massive than Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
. The temperature of the planet was initially estimated on the basis of 0.3 Bond albedo to be about 1540 K
Kelvin
The kelvin is a unit of measurement for temperature. It is one of the seven base units in the International System of Units and is assigned the unit symbol K. The Kelvin scale is an absolute, thermodynamic temperature scale using as its null point absolute zero, the temperature at which all...
, above the predicted temperature of HD 209458 b
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
(1400K) which had inaugurated the category of Chthonian
Chthonian planet
A chthonian planet is a hypothetical class of celestial objects resulting from the stripping away of a gas giant's hydrogen and helium atmosphere and outer layers, which is called hydrodynamic escape. Such atmospheric stripping is a likely result of proximity to a star...
"hell planet". 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
HD 209458 b
HD 209458 b is an extrasolar planet that orbits the Solar analog star HD 209458 in the constellation Pegasus, some 150 light-years from Earth's solar system, with evidence of water vapor....
. 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
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
of cooler gas is predicted. 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. Currently what limits more precision on HD 149026 b's radius "is the uncertainty in the stellar radius", and measurement of the stellar radius is distorted by pollution on the star's surface.
Even allowing for uncertainty the radius of HD 149026 b is only about three quarters that of Jupiter (or 83% that of Saturn). Only Gliese 436 b
Gliese 436 b
Gliese 436 b is a Neptune-sized extrasolar planet orbiting the red dwarf star Gliese 436. It was among the smallest known transiting planets in mass and radius until the much smaller Kepler discoveries started coming in 2010.-Discovery:...
of 52 planets known to transit their stars as of August 2008 is smaller in radius than HD 149026 b.
There are a number of such "hot Saturns
Hot Jupiter
Hot Jupiters are a class of extrasolar planet whose mass is close to or exceeds that of Jupiter...
", but HD 149026 b is so far unique: HD 149026 b's low volume means that the planet is unexpectedly dense for a gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
of its mass and temperature. It may have an exceptionally large core composed of elements heavier than hydrogen and helium: the initial theoretical models gave the core a mass of 70 times Earth's mass; further refinements suggest 80-110 Earth masses. 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. 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". In addition to uncertainties of radius, its tidal heating over its history needs be taken into account; if its current orbit had evolved from a more eccentric one, the extra heat increases its expected radius per its model and thereby its core radius.
Naeye further speculated that the gravity could be as high as ten g
G-force
The g-force associated with an object is its acceleration relative to free-fall. This acceleration experienced by an object is due to the vector sum of non-gravitational forces acting on an object free to move. The accelerations that are not produced by gravity are termed proper accelerations, and...
(ten times gravity on Earth's surface) on the surface of the core.
Theoretical consequences
The discovery was advocated as a piece of evidence for the popular solar nebulaSolar nebula
In cosmogony, the nebular hypothesis is the most widely accepted model explaining the formation and evolution of the Solar System. There is evidence that it was first proposed in 1734 by Emanuel Swedenborg. Originally applied only to our own Solar System, this method of planetary system formation...
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
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. 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.
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 creating the large core.
Careful radial velocity measurements have made it possible to detect the Rossiter-McLaughlin effect
Rossiter-McLaughlin effect
The Rossiter–McLaughlin effect is a spectroscopic phenomenon observed when either an eclipsing binary's secondary star or an extrasolar planet is seen to transit across the face of the primary or parent star. As the main star rotates on its axis, one quadrant of its photosphere will be seen to be...
, the shifting in photospheric
Photosphere
The photosphere of an astronomical object is the region from which externally received light originates. The term itself is derived from Ancient Greek roots, φῶς, φωτός/phos, photos meaning "light" and σφαῖρα/sphaira meaning "sphere", in reference to the fact that it is a spheric surface perceived...
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. As of August 2008 this is still the canonical measurement.