Exoplanets' Structures

Exoplanets' structures

In the coming decade, observational campaigns will help us further our understanding of planet structure, both within and outside of our solar system. The long-standing question of how much water there is in Jupiter should be answered thanks to the Juno mission, which will offer critical new insights into Jupiter's interior structure. The recently authorized Transiting Exoplanet Survey Satellite will locate several planets outside of our solar system that is located around stars that are close enough and bright enough to allow for follow-up studies from the ground or using the James Webb Space Telescope. Our knowledge of these planets' bulk compositions and architectures will be influenced by what we learn about their masses (through radial velocity measurements) and atmospheres.

Finally, in some circumstances, large exoplanets, like those in the HR 8799 system, that are sufficiently remote from their stars and sufficiently self-luminous, may be directly observed using modern, high-contrast imaging methods. These up-close studies of young objects are sensitive to the conditions of the planet’s birth, and as a result, they may be able to inform us about the internal structures of young, widely separated Jovian objects and help us discriminate between different formation processes. The Gemini Planet Imager, the Spectro-Polarimetric High-Contrast Exoplanet Research Instrument on the Very Large Telescope, and other ground-based programs are among those that will carry out these surveys. As a result of the unexpected structures that exoplanetary studies have revealed in both big and small planets, the potential of an influx of new data offers more surprises and fresh perspectives on comparative planetology.

Exoplanets, as they are known, are planets that orbit other stars than the sun. Exoplanets have a significantly larger variety of physical characteristics than the planets in our solar system, ranging from tight rocky planets with densities as high as iron to enormously puffy gas giants. The diversity of exoplanets encourages further study in the embryonic subject of comparative exoplanetology and enables us to see the planets of our solar system from a wider viewpoint. Here, we examine the range of discovered planets, from the gas giants, which are mostly made of hydrogen, to smaller planets, where water may account for a greater portion of their mass, to planets made of rock and iron that resemble Earth in some respects.

How can we learn about the internal architecture of exoplanets from a great distance away? When seen separately, none of these observations—which give planet radii and minimal exoplanet masses—is very illuminating regarding the structure of the planets. However, we could discover a lot more about a planet's internal structure if we knew both its mass and radius. As of early 2013, the space-based Kepler spacecraft had discovered over 100 planets and approximately 3,000 planet candidates, the great majority of which are very likely genuine. There are currently more than 200 confirmed planets with measured masses and radii, spanning a variety of irradiation conditions, thanks to Kepler and ground-based initiatives like the Hungarian Automated Telescope (HAT) and Super Wide Angle Search for Planets (SuperWASP) transit studies.

Large spheres of hydrogen and helium make up most of Jupiter and Saturn, with minor amounts of heavier elements and complex molecules. Numerous known exoplanets share mass and radius with our nearby gas giants and likely share a similar bulk structure as well. The atmosphere, or weather layer, of such an object, is a thin outer region that is about the same relative depth as the grapefruit skin. It is typically defined as the region above the radiative-convective boundary, which can be at pressures on the order of kilobars (kbar) for the most strongly irradiated planets and that occurs in the vicinity of 1 kbar in gas giants subjected to lower irradiation, such as Jupiter and Saturn. A deep envelope that almost completely encircles the globe and with opacities so high that heat must be transferred by convection exists below the radiative-convective barrier; this area is likely well-mixed in terms of its chemical makeup and specific entropy. It is unknown if all gas giants have cores, although some of them contain heavy-element nuclei at their centres.

Most structural models suggest that the bulk of the planetary mass is in a deep fluid ionic sea that is likely to be mostly composed of water, as well as having ammonia and methane. As a result, Uranus and Neptune are sometimes grouped together as two "ice giants" and are recognized for their bluish color. There is plenty of evidence that the inside of the planets is significantly diverse, despite the fact that they appear to be very similar to the outside. Our two Neptune-class planets' variety should serve as a constant reminder that exoplanets in this class should be incredibly diverse.

First, neither planet simply has a three-layer structure consisting of a rocky core, a middle envelope predominated by water, and an upper envelope made of Hydrogen and Helium. Neither planet is as centrally compressed as this frequently proposed but the overly simplistic image would suggest. More strikingly, Uranus has a heat flow from its deep interior that is just 10% that of Neptune, which may be caused by deep composition gradients that prevent large-scale convection. Uranus is more centrally condensed than Neptune. The fact that Uranus is likewise on its side and nearly has its spin axis in its orbital plane suggests that huge collisions at the end of the planet-formation epoch may have had a significant influence in establishing the structure of this class of planet. Much of our knowledge about the composition and development of these planets is still tentative and uncertain today.

The Hydrogen and Helium atmospheres of both planets are heavily loaded with metals. With a carbon enhancement of around 50 solars on each planet, spectroscopy can only reasonably and accurately detect the carbon abundance (in methane). The comparatively thin Hydrogen and Helium envelopes of Neptune-class planets may very possibly have significantly increased metal content compared to their parent stars. Indirect evidence implies that oxygen may be several hundred times solar in Neptune.

It is unclear if Neptune-class exoplanets are real ice giants, which means that a significant portion of their mass is made up of liquid planetary ice-like water. If, like all of our huge planets, planets develop beyond the ice line, where water has condensed, then a sizeable portion of the mass of Neptune-class planets is likely made up of water. But planets can also form within the ice line, in which case their interiors would be made of rock or iron and their gaseous envelopes would have formed over time.

A fresh viewpoint on planetary structure is provided by the hundreds of exoplanets that have been found in the last 20 years. Our solar system's planets are not the prototypical instances of planets; rather, they are only potential results of the creation and development of planetary systems, and maybe not even very typical ones (although this is still up for debate). Here, we review the wide range of interior structures that are both known and hypothesized to exist in exoplanetary systems, ranging from rocky objects with roughly the mass of Earth to mostly degenerate objects more than ten times as massive as Jupiter.