| Extrasolar planet | List of extrasolar planets | |
|---|---|---|
 | ||
| Parent star | ||
| Star | Gliese 436 | |
| Constellation | Leo | |
| Right ascension | (α) | 11h 42m 11.0941s[1] |
| Declination | (δ) | +26° 42′ 23.652″[1] |
| Apparent magnitude | (mV) | 10.68 |
| Distance | 33.4 ± 0.8 ly (10.2 ± 0.2 pc) | |
| Spectral type | M2.5 V[1] | |
| Mass | (m) | 0.41 ± 0.05 M☉ |
| Radius | (r) | 0.42 R☉ |
| Temperature | (T) | 3318 K |
| Metallicity | [Fe/H] | -0.32 |
| Age | 7.41–11.05[2] Gyr | |
| Orbital elements | ||
| Semimajor axis | (a) | 0.0291±0.0004[3] AU (4.35 Gm) |
| 2.85 mas | ||
| Periastron | (q) | 0.0247 AU (3.70 Gm) |
| Apastron | (Q) | 0.0335 AU (5.01 Gm) |
| Eccentricity | (e) | 0.150±0.012[3] |
| Orbital period | (P) | 2.643904±0.000005[4] d (0.00723849 y) |
| (63.4537 h) | ||
| Inclination | (i) | 85.8+0.21 −0.25[4]° |
| Argument of periastron | (ω) | 351±1.2° |
| Time of periastron | (T0) | 2,451,551.716 ±0.01 JD |
| Semi-amplitude | (K) | 18.68±0.8 m/s |
| Physical characteristics | ||
| Mass | (m) | 22.2±1.0[3] M⊕ |
| Radius | (r) | 4.327±0.183[3][5] R⊕ |
| Stellar flux | (F⊙) | 29.5 ⊕ |
| Density | (ρ) | 1.51 g cm−3 |
| Surface gravity | (g) | 1.18 g |
| Temperature | (T) | 712±36[3] |
| Discovery information | ||
| Discovery date | August 31, 2004 | |
| Discoverer(s) | Butler, Vogt, Marcy et al. | |
| Discovery method | Radial velocity, Transit | |
| Discovery site | California, USA | |
| Discovery status | Published | |
| Other designations | ||
Ross 905 b, GJ 436 b,[6] LTT 13213 b, GCTP 2704.10 b, LHS 310, AC+27:28217 b, Vyssotsky 616 b, HIP 57087 b, GEN# +9.80120068 b, LP 319-75 b, G 121-7 b, LSPM J1142+2642 b, 1RXS J114211.9+264328 b, ASCC 683818 b, G 147-68 b, UCAC2 41198281 b, BPS BS 15625-0002 b, G 120-68 b, 2MASSJ11421096+2642251 b, USNO-B1.0 1167-00204205 b, CSI+27-11394 b, MCC 616 b, VVO 171 b, CSI+27-11395 b, HIC 57087 b, NLTT 28288 b, Zkh 164 b, CSI+26-11395 b, [RHG95] 1830 b, GCRV 7104 b, LFT 838 b, PM 11395+2700 b
| ||
Discovered in August 2004, Gliese 436b orbits a red-dwarf star located in the constellation Leo, around 33.4 light-years away and has a very
The planet has an orbital period of just 2.6 Earth days and a mass approximately 23 times that of our home planet. It is categorized as a warm Neptune because it is much closer to its star (just about 4 million km away) than frigid Neptune is to the Sun.
Gliese 436b has an atmosphere leaves behind a gigantic trail of hydrogen, which is about 50 times the size of the parent star, Gliese 436. A phenomenon this large has never before been seen around such a small planet.
“What we can see is a large cloud of hydrogen gas absorbing the light from a red dwarf star as its exoplanet, Gliese 436b, passes in front. The cloud is created as of result of X-rays emitted from the red dwarf burning off Gliese 436 b 's upper atmosphere,” explained Dr Peter Wheatley of the University of Warwick, UK, a co-author on the study published in the journal Nature.
“The cloud forms a comet-like tail as a result of UV light coming from the star pushing on the hydrogen and causing it to spiral outwards.
Because the atmosphere of our planet blocks most UV light, the team needed a space telescope with Hubble’s UV capability and exquisite precision to find the Gliese 436b’s comet-like tail.
“You would have to have Hubble’s eyes. You would not see it in visible wavelengths. But when you turn the UV eye of Hubble onto the system, it’s really kind of a transformation, because the planet turns into a monstrous thing,” said study lead author Dr David Ehrenreich from the Observatory of the University of Geneva in Switzerland.
He added: “this cloud of hydrogen is very spectacular. Although the evaporation rate doesn’t threaten the planet right now, we know that the star was more active in the past. This means that the planet’s atmosphere evaporated faster during its first billion years of existence. Overall, we estimate that it may have lost up to 10 percent of its atmosphere.”
“Around 1,000 metric tons of hydrogen are being burnt off from Gliese 436b’s atmosphere every second; which equates to only 0.1 percent of its total mass every billion years,” Dr Wheatley said.
The same process may explain the disappearance of atmospheres observed on terrestrial exoplanets, which rotate very close to their star and are extremely hot, such as the so-called super-Earths.
“Finding the cloud around Gliese 436b could be a game-changer for characterizing atmospheres of the whole population of Neptunes and super-Earths in UV observations,” said co-author Dr Vincent Bourrier, also from the Observatory of the University of Geneva.
The astronomers suggest that evaporation such as this may also have happened in the earlier history of the Solar System, when the Earth had a hydrogen-rich atmosphere that dissipated.
It is also possible that it could happen to Earth’s atmosphere at the end of our planet’s life, when the Sun swells up to become a red giant and boils off our remaining atmosphere, before engulfing our planet completely.
Gliese 436 b /ˈɡliːzə/ (sometimes called GJ 436 b[7]) is a Neptune-sized exoplanet orbiting the red dwarf Gliese 436.[8] It was the first hot Neptune discovered with certainty (in 2007) and was among the smallest known transiting planets in mass and radius until the much smaller Kepler exoplanet discoveries started coming in by 2010.
In December 2013, NASA reported that clouds may have been detected in the atmosphere of GJ 436 b.[9][10][11][12]
Contents
[hide]Discovery[edit]
Gliese 436 b was discovered in August 2004 by R. Paul Butler and Geoffrey Marcy of the Carnegie Institute of Washington and University of California, Berkeley, respectively, using the radial velocity method. Together with 55 Cancri e, it was then the first of a new class of planets with a minimum mass (M sini) similar to Neptune.
The planet was recorded to transit its star by an automatic process at NMSU on January 11, 2005, but this event went unheeded at the time.[13] In 2007, Gillon led a team which observed the transit, grazing the stellar disc relative to Earth. Transit observations led to the determination of Gliese 436 b's exact mass and radius, both of which are very similar to Neptune. Gliese 436 b then became the smallest known transiting extrasolar planet. The planet is about 4000 km larger in diameter than Uranus and 5000 km larger than Neptune and a bit more massive. Gliese 436b (also known as GJ 436b) orbits its star at a distance of 4,000,000 km or 15 times closer than Mercury's average distance from the Sun.
Physical characteristics[edit]
The planet's surface temperature is estimated from measurements taken as it passes behind the star to be 712 K (439 °C).[3] This temperature is significantly higher than would be expected if the planet were only heated by radiation from its star (which had been, in a Reuters article from a month prior to this measurement, estimated at 520 K). Whatever energy that tidal effects deliver to the planet does not notably affect its temperature.[14] Its discoverers allowed for a temperature increase due to a greenhouse effect.[15]
Its main constituent was initially predicted to be hot "ice" in various exotic high-pressure forms,[15][16] which remains solid because of the planet's gravity despite the high temperatures.[17] The planet could have formed further from its current position, as a gas giant, and migrated inwards with the other gas giants. As it arrived in range, the star would have blown off the planet's hydrogen layer via coronal mass ejection.[18]
However when the radius became better known, ice alone was not enough to account for it. An outer layer of hydrogenand helium up to ten percent in mass would be needed on top of the ice to account for the observed planetary radius.[3][4] This obviates the need for an ice core. Alternatively, the planet may be a super-earth.[19]
Observations of the planet's brightness temperature with the Spitzer Space Telescope suggest a possible thermochemical disequilibrium in the atmosphere of this exoplanet. Results published in Nature suggest that Gliese 436b's dayside atmosphere is abundant in CO and deficient in methane (CH4) by a factor of ~7,000. This result is unexpected because, based on current models at this temperature, the atmospheric carbon should prefer CH4 over CO.[20][21][22][23]
In June 2015, scientists reported that the atmosphere of Gliese 436 b was evaporating, resulting in a giant cloud around the planet and, due to radiation from the host star, a long trailing tail 14×106 km (9×106 mi) long.[24]
Orbital characteristics[edit]
One orbit around the star takes only about 2 days, 15.5 hours. Gliese 436 b's orbit is likely misaligned with its star's rotation.[22]
The eccentricity of Gliese 436 b's orbit is inconsistent with models of planetary system evolution. To have maintained its eccentricity over time requires that it be accompanied by another planet.[3][26]
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