> [!Definition] > A **planet** is a celestial body that: > 1. Orbits its host star > 2. Is large enough to be mostly round ($M \lesssim 0.013 \; {\rm M_{\odot}}$) > 3. Has an important influence on the orbital stability of the other objects in its neighborhood ## Rocky Planets *(Also known as **terrestrial planets** or **telluric planets**.)* A smaller planets (${\rm radius} \in \left[ 0.5, \, 2 \right] \, {\rm R_{\oplus}}$) composed primarily of rock, silicate, metals, water and/or carbon. By the [[Standard Model of Planetary Formation]] theory, they tend to be planets closer to the star/protostar of a planetary system. Due to high stellar winds and temperatures, the "ices" were vaporized and pushed further out beyond their [[Standard Model of Planetary Formation#^ice-line|ice lines]]. Therefore, they were unable to form a gas envelope like the [[#Gas Giants]] and [[#Ice Giants]]. **Examples:** Mercury, Venus, Earth, Mars ## Gas Giants *(Also known as **failed stars**.)* A giant planet composed mainly of hydrogen and helium. After the initial rocky formation from clumping [[Smaller Non-Planetary Bodies#Planetesimals|planetesimals]], gravity pulled in the surround gas creating a thick gaseous envelope about the planet. With the high density, temperature, and pressure in the core, is it thought that gas giants are composed of a molten rocky core with the outer layers consisting of liquid metallic hydrogen followed by compressed molecular hydrogen. The outermost portion of their hydrogen atmosphere can contains visible clouds of water and ammonia. Gas giants share similarities with low-mass brown dwarf stars ($\sim 13 \; {\rm M_{Jup}}$), but the distinct differences are still debated. It could be related to their formation, the physics of the interior or if they have experienced nuclear fusion before. **Examples:** Jupiter, Saturn ## Ice Giants A giant planet are similar to [[#Gas Giants]]; however, they are mostly composed mainly of elements **heavier than** hydrogen and helium (oxygen, carbon, nitrogen, sulfur, etc.). When their rocky-icy cores started to develop their gaseous envelopes, most of the constituent compounds were heavier solids in the form of "ices". We see this difference between Ice Giants and Gas Giants from the position of the [[Standard Model of Planetary Formation#^ice-line|ice lines]] relative to the star/protostar. **Examples:** Neptune, Uranus ## Hot Jupiters *(Also known as **hot Saturns**)* A class of [[#Gas Giants]] that are inferred to be physically similar to Jupiter but that have very short orbital periods and high surface-atmosphere temperatures due to their close proximity to their stars. Hence, the name *"hot Jupiters".* The dicover of these planets is surprising because they are too large for their radial position with normal [[Standard Model of Planetary Formation|planetary formation theory]]. Due to the gravitational influence they have on their host star, they can cause the star to “wobble” (through tidal forces) and emit a measurable shifts in spectrum from the oscillatory motion. This makes Hot Jupiters very easy to detect via the [[Detection Methods#Radial Velocity Method|radial velocity method]]. - **Mass Range:** $M \in \left[ 0.36, \, 13 \right] \, {\rm M_{Jup}}$ - **Period Range:** $P \lesssim 10 \; {\rm days}$ - **Radial Position:** $R \lesssim 1 \; {\rm AU}$ > [!note] Maximum Hot Jupiter Mass > *The upper limit on the mass range is defined by the temperature and pressure needed for deuterium fusion. Any higher, and the planet would be considered a [[Stellar Classes#Brown Dwarf]].* #### Observed Characteristics of Hot Jupiters: - They tend to be [[Planetary Migration#Tidal Locking]] with the host star, such that only one side is always facing it - Most have nearly circular orbits (low eccentricities). - This is thought to be caused by perturbations from nearby stars or tidal forces. - Any major perturbations could cause the planet to dissipate or plunge into the host star. - Many have low densities. - It is predicted that the expanded envelopes on these planets (contributing to the low density) could be due to high stellar irradiation, high atmospheric opacities, internal energy sources, and/or the small orbital radius (allowing the outer layers of the planets to exceed their [[Question 48|Roche Limit]]). - They tend to be observed near [[Spectral Classes#F|F-type]] and [[Spectral Classes#G|G-Type]] stars (and a few [[Spectral Classes#K|K-Type]] stars). - Despite the extreme temperatures, the giant planets seem to have strong-enough gravity to keep hold of their gas. - Many detected hot Jupiters *(See [[Question 11]])* have high obliquity, suggesting their orbit may be due to dynamical interaction rather than simple orbit migration. #### Theorized Origins: There are two methods theorized to account for the existence of Hot Jupiters. One possibility is that they were formed in-situ - at the distances at which they are currently observed. Another possibility is that they were formed further out, where there was enough material to accrete into a Jupiter-sized planet, and later migrated inward. 1) In-Situ Formation - This is not expected origin for Hot Jupiters because giant planets are thought to form either by core accretion or gravitational instability in the disk (the protoplanetary disk fragments into bound clumps) - Core accretion can operate close to the star, but building a sufficiently large core ($\sim 10 \, {\rm M_{\oplus}}$) is challenging due to small feeding zones. - The local available solids are insufficient. - Mergers of multiple smaller cores are prevented by the disk - Accretion of radially transported pebbles stalls at a much lower mass. - Close to the star, gas conditions prevent formation by gravitational instability. 2) Migration - If the planets migrated slowly... *(not as likely due to high obliquity?)* - when the protoplanetary disk was still hot and filled with gas, [[Planetary Migration#Disk Migration]] ([[Planetary Migration#Type I]] & [[Planetary Migration#Type II]]) - when the planet had mostly formed, [[Planetary Migration#Tidal Migration]] - If the planets migrated quickly... - when the protoplanetary disk was still hot and filled with gas, [[Planetary Migration#Disk Migration]] ([[Planetary Migration#Type III]]) - and were sent on highly eccentric orbits by [[Planetary Migration#Gravitational Scattering]] or the [[Planetary Migration#Kozai Cycles and Tidal Friction|Kozai Mechanism]], they settled into shallow, circularized orbits by [[Planetary Migration#Tidal Migration]]. ## Eccentric Jupiters Giant planets with idiosyncratic or eccentric orbits. ## Super-Earths *(Also known as **sub-Neptunes** or **mini-Neptunes**)* Super-Earths are more massive than Earth yet lighter than ice giants like Neptune and Uranus. They are composed of of gas, rock or a combination of both. Although there are none in our Solar System and they tend to only be found in compact systems of two to four planets each, they are the most common type of planet found, orbiting $\sim 40 \%$ of all Sun-like stars. - **Mass Range:** $M \in \left[ 2, \, 10 \right] \, {\rm M_{\odot}}$ - **Orbital Range:** $R \in \left[ 0.006, \, 1 \right] \, {\rm AU}$ - **Period Range:** $P \in \left[ \sim 10 \, {\rm hour}, \, \sim 100 \, {\rm day} \right]$ > [!note] >The name is only a reference to the exoplanet’s size – larger than Earth and smaller than Neptune. It is not a reference to nature/composition of the planet itself. > >Within this mass range though, there is a wide variety of possible planetary compositions, including water worlds, snowball planets, or planets composed of dense gas. ## Dwarf Planet A body with characteristics similar to those of a classical planet with a nearly round shape; however, it has not been able to clear its orbit of debris. **Examples:** Pluto, [[Smaller Non-Planetary Bodies#Trans-Neptunian Object (TNO)]] --- ## Resources - [Wikipedia - List of Planets Types](https://en.wikipedia.org/wiki/List_of_planet_types)