Exoplanet interiors

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An artist rendering that compares the sizes of a few known exoplanets with the Moon, Mercury, Mars, and Earth, illustrating the diversity in observed exoplanets. Exoplanet diversity.jpg
An artist rendering that compares the sizes of a few known exoplanets with the Moon, Mercury, Mars, and Earth, illustrating the diversity in observed exoplanets.

Over the years, our ability to detect, confirm, and characterize exoplanets and their atmospheres has improved, allowing researchers to begin constraining exoplanet interior composition and structure. While most exoplanet science is focused on exoplanetary atmospheric environments, the mass and radius of a planet can tell us about a planet's density, and hence, its internal processes. The internal processes of a planet are partly responsible for its atmosphere, and so they are also a determining factor in a planet's capacity to support life.

Contents

Methods

Because humans cannot travel to exoplanets and take direct measurements, scientists must use other methods to constrain their interior composition. Measuring a planet's mass and radius - characteristics that can be observed - can give clues to its internal structure. We can infer these characteristics from both the transit method and radial velocity method. As a planet transits in front of its star, the planet's radius can be determined from the reduction in light from the star. Similarly, because the planet's pull on the star causes the star to "wobble", we can use the resulting Doppler shifts of starlight to infer the mass of the planet relative to its parent star. Within the Solar System, rocky planets have a mantle and crust containing silicates, oxides, and silicate melts, and an iron-rich core. [1] The principal elements that compose rocky planets, which are magnesium, iron, oxygen, carbon, and silicon, are assumed to be universal in the interiors of these planet types. [2] The abundance of these elements, as well as the heating and cooling processes they undergo during planet formation, are responsible for the planet's final composition. By measuring the mass and radius of exoplanets, the mean density of the planet can be calculated. Dense planets must have a higher proportion of heavier elements such as iron, whereas lighter planets must have higher concentrations of lighter elements such as hydrogen. [3] However, mean density does not distinguish how different components are distributed within a planet's interior; a dense planet might have its dense material packed into a small core, or distributed through its mantle.

In addition to analyzing mass-radius relationships of planets, researchers also look to the composition of a planet's host star when hypothesizing a planet's interior composition. This is because planets and their host stars originate from the same system, so they share the same material from the accretion disk. [4] [5] Although planets will not have the exact same composition as their host star, they would share similar fractions of certain elements. For example, in metal-poor areas of the galaxy containing metal-poor stars, researchers assume that planets also have less metal abundance compared to systems and stars that are richer in metals. The iron abundance of a system is particularly important since it is a common component in planetary interiors, along with elements such as nickel and metallic alloys. [6] Iron is a relatively heavy element that is believed to exist universally in rocky planets. This iron can exist in the mantle in silicates and oxides if it is oxidized, but otherwise will form a terrestrial planet's core as a metal. Hence, the availability of oxygen, along with similarities in mass and radius to known rocky planets, can help indicate the possibility of a mantle with oxidized iron. [7]

Schematic representation of five possible interior compositions of terrestrial exoplanets (not an exhaustive list of possibilities). The figure was created by Tim Van Hoolst, Lena Noack, and Attilio Rivoldini, and appears in their publication "Exoplanet interiors and habitability" Interiors.jpg
Schematic representation of five possible interior compositions of terrestrial exoplanets (not an exhaustive list of possibilities). The figure was created by Tim Van Hoolst, Lena Noack, and Attilio Rivoldini, and appears in their publication "Exoplanet interiors and habitability"

Possible interior types

The Solar System has a variety of planetary interiors; Earth has an inner and outer core (reaching to approximately 55% of Earth's radius), mantle, and crust, and Venus is commonly thought to have a similar structure. [8] Mercury has a much larger core radius that 80% of the planetary radius, [9] as well as a mantle that is much more abundant in sulfur and much less abundant in iron relative to the other terrestrial planets. [10] Mars has a relatively smaller core, and a mantle containing roughly twice the mass fraction of iron compared to the Earth. [11] The cores of the outer planets Jupiter, Saturn, Uranus, and Neptune are much less understood, though it is believed Jupiter and Saturn have cores containing iron and nickel at temperatures and pressures far higher than those seen in the interiors of the inner, rocky planets. [12] In addition, the interiors of the outer planets have much more ice and volatiles relative to the inner rocky planets.

Because the planets in the Solar System contain a diverse set of interiors, the interiors of exoplanets likely exhibit similar or even greater diversity. For example, the thickness of a planetary crust is directly proportional to how quickly the planet cooled after its formation. A fast cooling rate is expected for a smaller planet, a low-mass planet, or a planet that is further away from its star. Such planets would have a proportionally thicker crust, as is seen in the Moon and Mars. [13]

However, there are types of planets that are not seen in the Solar System, such as low-density "hot Jupiters", which are hotter and larger than the Solar System's gas giants. [14] Observers have also found many "super-Earths" or "sub-Neptunes", i.e. planets with a radius between that of Earth and Neptune, [15] whose radius is 4 times that of Earth. The existence of planet types not represented in the Solar System suggests the existence of planetary interiors that are likewise not represented in the Solar System. Some proposed interior types include tiny cores, massive mantles, and an ocean surface with no continents: or, a massive core and mantle, or perhaps there can be planets with an ice layer between its mantle and surface, both above a core.

There are no confirmed examples of these proposed interior structures. However, there are hypotheses of the potential internal structures of some exoplanets. One such example is GJ 1214b, that has a mass 6.55 times greater than the Earth's and is 2.68 times the radius of Earth, but only approximately one-third Earth's density. GJ 1214b's density of1870 kg m−3 is too low to represent a dominantly metal and silicate rock composition, despite the large size of the planet. [16] Three different types of interior makeup have been hypothesized for this planet. [17] One is that the planet has an iron core, a mantle composed of silicates, and an outermost layer of water, which supports a thick envelope of hydrogen and helium. Another hypothesis keeps the iron core and silicate mantle assumption, but eliminates the water outer layer, and requires a thicker hydrogen/helium envelope. A third hypothesis proposes that the planet is mostly water, and has a large atmosphere composed mostly of steam.

As with GJ 1214b, there are many speculations but no confirmed compositions of exoplanets. As more exoplanets are discovered, the range of types of planets also grows, beyond planet types not seen in the Solar System. Some exoplanets may resemble the Earth in radius, but are much less dense or much denser. Othe rexoplanets can resemble Earth in density, but have a very different size. As such, it is difficult to classify potential planetary interiors based on their similarities to Solar System objects.

Continued exoplanet discoveries allow a statistical analysis of planet types, and the identification of distinct planetary populations of certain mass and radii. This allows an expansion of the definition of categories such as "terrestrial" planets or "gas giants" beyond that seen in the Solar System, to the range of all known exoplanets. [13]

Habitability

An artist rendering that of possible interiors of the TRAPPIST-1 system of rocky planets, which are targets of astrobiological interest. Possible Interiors of the TRAPPIST-1 Exoplanets.jpg
An artist rendering that of possible interiors of the TRAPPIST-1 system of rocky planets, which are targets of astrobiological interest.

The relevance of planetary-scale cycles to habitability drives much research into exoplanetary interiors. On Earth, surface and sub-surface processes such as plate tectonics, outer core convection powering the geodynamo which creates our magnetic field, and the cycling of material between the surface and interior contribute to the climate and atmospheric composition of Earth, and hence, its habitability. [18] Because of this, the understanding of planetary interiors is becoming more popular as the study of planetary habitability grows as well. Earth is the only known planet to harbor life, so in the search for life in exoplanets, many researchers focus on searching for conditions of early or modern Earth, [19] in particular the presence of atmospheric oxygen. [20] Similarly, in the search for habitable planets, those that are similar to Earth (mass, radius, interior structure) are of special interest.

In addition, a planet's size and mass contribute to whether it can retain its atmosphere. Planets that are less massive have a weaker gravitational pull and are less able to retain an atmosphere. Life as we know it requires an atmosphere, so planets that have the right composition to achieve adequate planetary mass are key in determining the habitability of the planet. The presence of a magnetic field also contributes to a planet's habitability, and signatures of a possible exoplanetary magnetic field have been detected. [21] Continued atmospheric observations that search for a magnetic field, when paired with calculations of the interior compositions of exoplanets, may provide further insight into the interior structure of an exoplanet.

Related Research Articles

<span class="mw-page-title-main">Exoplanet</span> Planet outside the Solar System

An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917 but was not recognized as such. The first confirmation of detection occurred in 1992. A different planet, initially detected in 1988, was confirmed in 2003. As of 1 February 2023, there are 5,307 confirmed exoplanets in 3,910 planetary systems, with 853 systems having more than one planet. The James Webb Space Telescope (JWST) is expected to discover more exoplanets, and also much more about exoplanets, including composition, environmental conditions and potential for life.

<span class="mw-page-title-main">Giant planet</span> Planet much larger than the Earth

The giant planets constitute a diverse type of planet much larger than Earth. They are usually primarily composed of low-boiling-point materials (volatiles), rather than rock or other solid matter, but massive solid planets can also exist. There are four known giant planets in the Solar System: Jupiter, Saturn, Uranus and Neptune. Many extrasolar giant planets have been identified orbiting other stars.

<span class="mw-page-title-main">Terrestrial planet</span> Planet that is composed primarily of silicate rocks or metals

A terrestrial planet, telluric planet, solid planet, or rocky planet, is a planet that is composed primarily of silicate rocks or metals. Within the Solar System, the terrestrial planets accepted by the IAU are the inner planets closest to the Sun: Mercury, Venus, Earth and Mars. Among astronomers who use the geophysical definition of a planet, two or three planetary-mass satellites – Earth's Moon, Io, and sometimes Europa – may also be considered terrestrial planets; and so may be the rocky protoplanet-asteroids Pallas and Vesta. The terms "terrestrial planet" and "telluric planet" are derived from Latin words for Earth, as these planets are, in terms of structure, Earth-like. Terrestrial planets are generally studied by geologists, astronomers, and geophysicists.

<span class="mw-page-title-main">Planetary core</span> Innermost layer(s) of a planet

A planetary core consists of the innermost layers of a planet. Cores may be entirely solid or entirely liquid, or a mixture of solid and liquid layers as is the case in the Earth. In the Solar System, core sizes range from about 20% to 85% of a planet's radius (Mercury).

<span class="mw-page-title-main">Carbon planet</span> Theoretical type of planet that contains more carbon than oxygen

A carbon planet is a theoretical type of planet that contains more carbon than oxygen. Carbon is the fourth most abundant element in the universe by mass after hydrogen, helium, and oxygen.

<span class="mw-page-title-main">Exomoon</span> Moon beyond the Solar System

An exomoon or extrasolar moon is a natural satellite that orbits an exoplanet or other non-stellar extrasolar body.

<span class="mw-page-title-main">Ocean world</span> Planetary body that includes a significant amount of water or other liquid

An ocean world, ocean planet, panthalassic planet, maritime world, water world or aquaplanet, is a type of planet that contains a substantial amount of water in form of oceans, either beneath the surface, as subsurface oceans, or on the surface with a hydrosphere, potentially submerging all dry land. The term ocean world is also used sometimes for astronomical bodies with an ocean composed of a different fluid or thalassogen, such as lava, ammonia or hydrocarbons like on Titan's surface.

<span class="mw-page-title-main">Super-Earth</span> Planet with a mass between Earth and Uranus

A super-Earth is an extrasolar planet with a mass higher than Earth's, but substantially below those of the Solar System's ice giants, Uranus and Neptune, which are 14.5 and 17 times Earth's, respectively. The term "super-Earth" refers only to the mass of the planet, and so does not imply anything about the surface conditions or habitability. The alternative term "gas dwarfs" may be more accurate for those at the higher end of the mass scale, although "mini-Neptunes" is a more common term.

This page describes exoplanet orbital and physical parameters.

<span class="mw-page-title-main">CoRoT-7b</span> Hot Super-Earth orbiting CoRoT-7

CoRoT-7b is an exoplanet orbiting the star CoRoT-7 in the constellation of Monoceros, 489 light-years from Earth. It was first detected photometrically by the French-led CoRoT mission and reported in February 2009. Until the announcement of Kepler-10b in January 2011, it was the smallest exoplanet to have its diameter measured, at 1.58 times that of the Earth and the first potential extrasolar terrestrial planet to be found. The exoplanet has a very short orbital period, revolving around its host star in about 20 hours.

<span class="mw-page-title-main">Discoveries of exoplanets</span> Detecting planets located outside the Solar System

An exoplanet is a planet located outside the Solar System. The first evidence of an exoplanet was noted as early as 1917, but was not recognized as such until 2016; no planet discovery has yet come from that evidence. What turned out to be the first detection of an exoplanet was published among a list of possible candidates in 1988, though not confirmed until 2003. The first confirmed detection came in 1992, with the discovery of terrestrial-mass planets orbiting the pulsar PSR B1257+12. The first confirmation of an exoplanet orbiting a main-sequence star was made in 1995, when a giant planet was found in a four-day orbit around the nearby star 51 Pegasi. Some exoplanets have been imaged directly by telescopes, but the vast majority have been detected through indirect methods, such as the transit method and the radial-velocity method. As of 1 February 2023, there are 5,307 confirmed exoplanets in 3,910 planetary systems, with 853 systems having more than one planet. This is a list of the most notable discoveries.

<span class="mw-page-title-main">Kepler-62f</span> Super-Earth orbiting Kepler-62

Kepler-62f is a super-Earth exoplanet orbiting within the habitable zone of the star Kepler-62, the outermost of five such planets discovered around the star by NASA's Kepler spacecraft. It is located about 990 light-years from Earth in the constellation of Lyra.

<span class="mw-page-title-main">Kapteyn b</span> Goldilocks super-Earth exoplanet orbiting Kapteyns Star

Kapteyn b is a candidate exoplanet that orbits within the estimated habitable zone of the red subdwarf Kapteyn's Star, located approximately 12.8 light-years from Earth. It was the closest potentially habitable exoplanet to the Solar System other than Tau Ceti e up until 2016, when Proxima Centauri b at 4.22 light-years was confirmed. It was pushed into fourth when Ross 128 b was confirmed in 2017.

<span class="mw-page-title-main">Gas giant</span> Giant planet mainly composed of light elements

A gas giant is a giant planet composed mainly of hydrogen and helium. Gas giants are also called failed stars because they contain the same basic elements as a star. Jupiter and Saturn are the gas giants of the Solar System. The term "gas giant" was originally synonymous with "giant planet". However, in the 1990s, it became known that Uranus and Neptune are really a distinct class of giant planets, being composed mainly of heavier volatile substances. For this reason, Uranus and Neptune are now often classified in the separate category of ice giants.

<span class="mw-page-title-main">Geodynamics of terrestrial exoplanets</span>

The discovery of extrasolar Earth-sized planets has encouraged research into their potential for habitability. One of the generally agreed requirements for a life-sustaining planet is a mobile, fractured lithosphere cyclically recycled into a vigorously convecting mantle, in a process commonly known as plate tectonics. Plate tectonics provide a means of geochemical regulation of atmospheric particulates, as well as removal of carbon from the atmosphere. This prevents a “runaway greenhouse” effect that can result in inhospitable surface temperatures and vaporization of liquid surface water. Planetary scientists have not reached a consensus on whether Earth-like exoplanets have plate tectonics, but it is widely thought that the likelihood of plate tectonics on an Earth-like exoplanet is a function of planetary radius, initial temperature upon coalescence, insolation, and presence or absence of liquid-phase surface water.

<span class="mw-page-title-main">K2-18b</span> Mini-Neptune orbiting the red dwarf K2-18

K2-18b, also known as EPIC 201912552 b, is an exoplanet orbiting the red dwarf K2-18, located 124 light-years (38 pc) away from Earth. The planet, initially discovered with the Kepler space telescope, is about eight times the mass of Earth, and is thus classified as a super Earth or a Mini-Neptune, and, as well, may be considered a hycean planet. It has a 33-day orbit within the star's habitable zone.

HD 219134 g, also known as HR 8832 g, is an unconfirmed exoplanet orbiting around the K-type star HD 219134 in the constellation of Cassiopeia. It has a minimum mass of 11 or 15 Earth masses, suggesting that it is likely a Neptune-like ice giant. Unlike HD 219134 b and HD 219134 c it is not observed to transit and thus its radius and density are unknown. If it has an Earth-like composition, it would have a radius 1.9 times that of Earth. However, since it is probably a Neptune-like planet, it is likely larger.

Diana Valencia is a Colombian planetary scientist and astrophysicist. She is an associate professor of Physics and Astrophysics, University of Toronto, Scarborough, and of Astronomy & Astrophysics, University of Toronto.

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