Hot Jupiter

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Artist's impression of the exoplanet HD 209458b (Osiris) in front of its star

Hot Jupiter ( German : Hot Jupiter ) refers to a class of exoplanets whose mass roughly  corresponds to that of Jupiter (1.9 × 10 27 kg) or exceeds this, and whose surface temperature is significantly higher than that of Jupiter (165 K, i.e. - 108 ° C). A minimum temperature for a classification in this planetary class is not generally specified; In the Sudarsky classification , the term is used for gas planets with an equilibrium temperature from 900 K (about 630 ° C).

The comparatively high surface temperature of the Hot Jupiters is due to the fact that - in contrast to the conditions in our solar system - they do not orbit their central star at an average distance of 5  astronomical units , but typically only about 0.05 AU (about 1/8 the distance between Mercury and the Sun ). The orbital period of the Hot Jupiters is between one and five days, with their mass rarely exceeding two Jupiter masses.

Examples are 51 Pegasi b (Dimidium), HD 209458 b (Osiris) and the exoplanets in the systems HD 195019 , HD 189733 and WASP-12b .

Calculated temperatures of exoplanets with masses between 0.1 and 10 Jupiter masses, for which such data were available by the end of May 2015
Hot Jupiters (along the left edge), which were discovered up to and including August 31, 2004. Red points: discovered by transit. Blue dots: discovered by measuring the radial velocity. Lines indicate limits of individual detection methods: transit, Doppler shift, astrometry and microlensing. Courtesy NASA / JPL-Caltech .

Discovery opportunities

Hot Jupiters are those exoplanets that are easiest to spot by measuring their radial velocity . Because due to their close orbit and their high mass they cause a very fast and strong oscillation of the central star in comparison to other planets .

In addition, the probability of observing a passage from Earth is considerably higher than that of planets with extended orbits , e.g. B. higher than Jupiter analogs .

Therefore, the majority of the exoplanets with a mass similar to Jupiter, which have been discovered to date (as of May 2015) and for which a usable temperature value can be derived from the measurement data, fall into the class of Hot Jupiter .

properties

Hot Jupiters have a few things in common:

  • Due to the strong insulation (solar radiation) they have a lower density than would otherwise be the case. This has an impact on the determination of the diameter, because the edge darkening during transit makes it more difficult to determine the entry and exit limits.
  • Their orbits have a low orbital eccentricity . Such planets synchronize their rotation with the period around the central star and therefore always show it the same side ( bound rotation ).
  • They occur in the F, G and K dwarfs close to the sun only with a probability of 1.2% and are therefore quite rare. In contrast, around 25% of the metal-rich stars near the Sun have exoplanets.
  • Hot Jupiters have a very low probability of being found around subgiants . Such stars are the first phase of development after the F, G, and K dwarfs leave the main sequence and transform into red giants due to cup burning . The Hot Jupiters are likely to be destroyed by tidal forces .
  • The orbital plane of Hot Jupiter is often not in the plane of rotation of the star, i.e. H. there is a slope . This can with the help of star spots are observed to move slowly across the surface of the star: it is namely to cover such a patch by a planet, this leads to an increase in the observed overall brightness as the planet instead of a part of the light Star surface now only blocks the light emanating from the darker star spot. If the axis of rotation of the star and the orbital plane of the planet were aligned with each other, these occultations would be repeated. This is usually the case with other exoplanets, while it is rare in Hot Jupiters. Therefore the orbit of Hot Jupiter might have been influenced by scattering with other planets, since it is assumed that all planetary orbits lie in the plane of rotation of their central star when they are formed.
  • Some Hot Jupiters orbit their star at a distance of only one star radius. These exoplanets are surrounded by extensive gas clouds that extend over the Roche Boundary Volume . The gas planets are stellar winds ablative eroded , and the intense radiation heated their atmosphere so much that the Brownian motion , the gravitational potential exceeds the planet.
  • With orbital radii of less than 0.08 AU, the diameter of the Hot Jupiter is considerably larger than would be expected from the incidence of electromagnetic radiation. Either the planets store heat very well for reasons that are not known, or there is an additional unknown energy source with an output of up to 10 27  erg / s.
  • Hot Jupiters in their narrow orbits increase the speed of their star's rotation due to tidal effects. The higher rotation speed in turn increases the magnetic activity of the star in the form of star spots and flares . This complicates the observation of Hot Jupiter and the determination of the age of the planetary systems , since the rotation speed of single stars is a good indicator of age.

development

Theoretical calculations suggest that all gas giants, including the Hot Jupiters , form near the ice line , which for most stars is a few astronomical units apart. It is assumed that the Hot Jupiters only reached their current orbit later ( migration ), as there was not enough material available at such a short distance to the central star to form planets of this mass in situ . This is supported by observations that no Hot Jupiters are found in young stars shortly after the protoplanetary disk has dissolved (not enough time for migration).

Due to the above Orbit inclination is also assumed that the Hot Jupiters were scattered out of their original orbit through interaction with the protoplanetary disk or with other planets, thus initiating migration. The resulting highly elliptical orbit is then circularized by tidal forces.

Alternative approaches assume that the gas planets lose orbital angular momentum due to friction in the protoplanetary disk and migrate inward. This movement comes to a standstill in a narrow orbit around the central star because the inner area of ​​the disk has already been freed of material in young stellar objects or because tidal waves between the star and the planet prevent further approach.

It is likely that many of the current orbits of hot Jupiter are not stable over the long term. Due to the Darwin instability or the Kozai effect , the gas planets could later merge with the central star, which could be observed as a luminous red nova . The estimated rate of a merger burst from a hot Jupiter is one event every 10 years in the Milky Way .

The physical properties of the Hot Jupiter are quite different. In particular, some have large radii and low average densities, while others have a dense core . This diversity could be the result of collisions of the gas planet with earth-like rock planets . Such planets could be picked up during the migration into its narrow orbit, and the energy released during the collision would lead to a sharp increase in the radius of the gas planet. If the remains of the rocky planet sink into the core of the gas planet, the stronger gravitational force leads to a contraction after the planet's atmosphere has cooled down.

See also

Individual evidence

  1. ↑ Mathias Scholz: Planetology of extrasolar planets. Berlin / Heidelberg 2014, ISBN 978-3-642-41748-1 , pp. 276/277
  2. ↑ a b database on exoplanet.eu , accessed on May 27, 2015
  3. ↑ JT Wright et al .: THE FREQUENCY OF HOT JUPITERS ORBITING NEARBY SOLAR-TYPE STARS . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.2273v1 .
  4. ↑ Kevin C. Schlaufman, Joshua N. Winn: EVIDENCE FOR THE TIDAL DESTRUCTION OF HOT JUPITERS BY SUBGIANT STARS . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1306.0567v1 .
  5. ^ R. Sanchis-Ojeda, JN Winn, DC Fabrycky: Starspots and spin-orbit alignment for Kepler cool host stars . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1211.2002v1 .
  6. ^ CA Haswell et al .: Near-UV Absorption, Chromospheric Activity, and Star-Planet Interactions in the WASP-12 system. In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1301.1860 .
  7. ↑ D. Buzasi: STELLAR MAGNETIC FIELDS AS A HEATING SOURCE FOR EXTRASOLAR GIANT PLANETS . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1302.1466v1 .
  8. ↑ K. Poppenhaeger, SJ Wolk: Planets spinning up their host stars: a twist on the age-activity relationship . In: Astrophysics. Solar and Stellar Astrophysics . 2013, arxiv : 1309.6356v1 .
  9. ^ Jason H. Steffen et al .: Kepler constraints on planets near hot Jupiters . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1205.2309v1 .
  10. ↑ BD Metzger, D. Giannios, DS Mirror: Optical and X-ray Transients from Planet star mergers . In: Astrophysics. Solar and Stellar Astrophysics . 2010, arxiv : 1204.0796 .
  11. ↑ Benjamin J. Shappee, Todd A. Thompson: THE MASS-LOSS INDUCED ECCENTRIC KOZAI MECHANISM: A NEW CHANNEL FOR THE PRODUCTION OF CLOSE COMPACT OBJECT-STELLAR BINARIES. In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1204.1053v1 .
  12. ^ Kassandra R. Anderson, Fred C. Adams: Effects of Collisions with Rocky Planets on the Properties of Hot Jupiters . In: Astrophysics. Solar and Stellar Astrophysics . 2012, arxiv : 1206.5857v1 .