Classification of the planets

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This article covers the classification of planets in astronomy . This branch is currently part of intensive research, which is why new planetary classes are constantly being proposed, which, however, as of today (2019) cannot yet be divided into a basic scheme.

introduction

The classification of planets is only in the making, after traditionally only the planets of the solar system could be classified. The oldest classification knew only two categories, the rock planets and the gas planets (gas giants). Originally the term gas planet also included Uranus and Neptune , but today many astronomers include Uranus and Neptune in their own class of ice giants .

Several thousand exoplanets are now known, but that is only a fraction of the number of known stars . It is also still very difficult to measure the parameters required for a more precise determination.

Experimentally, most of the properties can best be determined by combining a measurement using the transit method and the radial velocity method. In some exoplanets, it has even been possible to determine certain components of the atmosphere (see also WASP-12b ).

The following properties currently lead to a classification into a certain type of planet:

Measurement principles

Suggested classifications

PHL classification

The Planetary Habitability Laboratory (PHL) is a research project aiming to assess the habitability of the solar system and exoplanets. The PHL is supervised by the University of Puerto Rico at Arecibo .

The classification of the PHL is currently (2019) based on three predictable properties of exoplanets. This classification is referred to by the institution itself as the Periodic Table of Exoplanets .

  • the spectral class of the central star
  • the position of the planet in the corresponding solar system
  • the size of the planet

temperature

The first two properties, spectral class and position of the planet, are combined to give a statement about the equilibrium temperature of the planet with an assumed bond albedo of 0.3 (earth = 255 K ). The effective surface temperatures are assumed to be higher due to the atmosphere ( greenhouse effect ). The global surface temperature of the earth, for example, at 288 K and 15 ° C, about 30 K higher. The three temperature-dependent classes arise, whereby only the "warm zone" is to be understood as a habitable zone . (The spectral class of the star is also specified, as red dwarfs , for example, tend to flare and stars that are too large have too short a lifespan.)

  • Hot, warm and cold

size

The size of the planet leads to a classification either according to its mass and / or its radius . 6 size classes are defined:

PHS size classes
Class name comment Area of ​​mass ( M ) Area of ​​the radius ( R )
Miniterran approx. size of Mercury 10 -5 to 0.1 0.03 to 0.4
Subterrans approx Mars size 0.1 to 0.5 0.4 to 0.8
Terrans approx earth size 0.5 to 5 0.8 to 1.5
Super trans Super-earths and mini-Neptune 5 to 10 1.5 to 2.5
Neptunians approx. Neptune size 10 to 50 2.5 to 6
Jovians approx. Jupiter size > 50 > 6

Emerging PHL classes

The Earth , in this system as a G-hot-Terran are divided (see list of potentially habitable planets ). The next exoplanet Proxima b , which is very similar to Earth according to previous knowledge, falls into the M-Warm-Terran category , since Proxima Centauri, as a red dwarf, only has the spectral class M.

Example of a classification in a 2019 study

In a study by Tuomi et al. the planets were only classified based on their maximum mass. This is due to the fact that the radial velocity method usually only allows a statement to be made about the maximum mass. In addition, it was assessed whether the planets are within the habitable zone (HZ) or in the hotter (H) or colder (C) areas.

Size class
Class name Area of ​​mass ( M )
Similar to Earth <2
Super-earth <10
Mini-Neptune (mini-Neptune) <Neptune mass (~ 20)
Neptune (Neptune) <Saturn mass (~ 100), roughly in the range of Neptune mass
Super neptune (super neptune) <Saturn mass, but certainly more massive than Neptune
Giant <13 M J , so in any case smaller than a brown dwarf

Important planet classes

In addition to the classic rock planets, gas giants and ice giants already known from the solar system, the observations of exoplanets have led to some new discoveries. The most important of these are the following.

Hot Jupiter

The Hot Jupiters differ from the common gas giants mainly in their extreme proximity to the star. This proximity to the stars makes them particularly easy to detect using the radial velocity method, which is why they dominated at the beginning of the exoplanet discovery. Hot Jupiters often have extremely short orbital times of a few days and are also characterized by a relatively low density. The low density is the result of the extreme temperatures on these planets, causing them to expand.

Hot Neptune

The Hot Neptunes are very similar to the Hot Jupiters . The biggest difference is their significantly lower mass. They are also relatively easy to detect due to their close proximity to the star.

Super earth

The super-earths are a new type of planet not known to our solar system. They are characterized by their mass, which on the one hand significantly exceeds that of the earth, but on the other hand is below that of Uranus . It is believed that most of the super-earths are still rocky planets, with some specimens being verified by density confirmation. If such a planet falls well below the necessary density, one speaks of a mini-Neptune , ocean planet or gas dwarf . The Kepler-138 system contains two such exoplanets and a super-earth. Due to even higher mass combined with high density, the term mega-earth was proposed after the discovery of Kepler-10c .

Example planets

planet Star system Mass ( M ) Planetary class comment
earth Sun 1 Earth-like planet -
Jupiter Sun 318 Gas planet -
51 Pegasi b 51 pegasi 150 Hot Jupiter first discovered exoplanet around a sun-like star
Proxima Centauri b Proxima Centauri 1.3 Earth-like planet next known exoplanet
Gliese 436 b Peter 436 22nd Hot Neptune -
Gliese 667Cc Gliese 667C 3.8 Super earth is in the habitable zone

Web links

Individual evidence

  1. Nikku Madhusudhan, Heather Knutson, Jonathan Fortney, Travis Barman: Exoplanetary Atmospheres . In: Henrik Beuther, Ralf S. Klessen, Cornelis P. Dullemond, Thomas Henning (Eds.): Protostars and planets . tape VI . The University of Arizona Press, Lunar and Planetary Institute, Tucson / Houston 2014, ISBN 978-0-8165-3124-0 , pp. 739–762 , doi : 10.2458 / azu_uapress_9780816531240-ch032 , arxiv : 1402.1169 , JSTOR : j.ctt183gxt8 .
  2. PHL: (HEC) Periodic Table of Exoplanets. University of Puerto Rico , accessed June 15, 2019 .
  3. M. Tuomi, HRA Jones, G. Anglada-Escudé, RP Butler, P. Arriagada, SS Vogt, J. Burt, G. Laughlin, B. Holden, JK Teske, SA Shectman, JD Crane, I. Thompson, S. Keizer, JS Jenkins, Z. Berdiñas, M. Diaz, M. Kiraga, JR Barnes: Frequency of planets orbiting M dwarfs in the solar neighborhood . In: arxiv . 2019. arxiv : 1906.04644 .
  4. Exoplanet.eu catalog , accessed on February 10, 2018.
  5. Exoplanet.eu catalog , accessed on February 10, 2018.
  6. Exoplanet.eu catalog , accessed June 15, 2019.
  7. Exoplanet.eu catalog , accessed on February 10, 2018.