extra-solar planet

An extra-solar planet (or exoplanet) is a planet outside the solar system, e.g., orbiting another star. The first confirmed detection of an exoplanet was in 1992 (PSR 1257+12 A) and the first orbiting a main sequence star (51 Pegasi) in 1995. As of 3/2024, 5500+ planets around 4200+ stars are known. Detection methods:

• radial velocity of the star caused by the orbit of the planet, detected via Doppler shift (radial velocity method or RV method). This requires a degree of brightness (e.g., an apparent magnitude of +12 or brighter).
• transit: brightness changes consistent with transits (a transiting planet, the transit method).
• direct imaging: observing the planet.
• transit timing variations, i.e., after a planet's transits are detected, variations in the transit timing can reveal the existence of another planet.
• gravitational microlensing: a star with a planet magnifying a further star can produce a recognizable pattern of magnification. Easiest detection is for orbits on the order of 2 AU radius.
• astrometry: deducing the presence of a planet from the star's detected motion.
• pulsar timing: pulse reception-timing varying due to the varying distance to the pulsar, caused by an orbiting planet.
• circumstellar disks: dust near a star that suggests an impacts of planets.
• circumstellar disks: a gap in the disk such as a planet might create.
• circumstellar disks: a clump in the disk suggesting a planet's gravity.

Note that commonly one of these methods turns up a candidate, and subsequent detection by a second method is taken as confirmation of a discovery and reports of such a discovery may vary regarding which of the two methods was that used to discover the planet. For example, surveys for transits can produce many candidates, often confirmed using the RV method.

A number of methods have been contemplated for future technology:

• detecting the effects of magnetism, e.g., the planet's magnetic field's effect on the star's field.

The RV method reveals the relative masses of the host star and planet, and a transit reveals their relative radius, so the density of a planet can be estimated if the planet can be observed by both methods, limited by the accuracy of these estimates for the star. For this reason, follow-ups are carried out to detect it by both if that possibility looks promising, leading to attempts to predict transit times from RV data, especially if the orbit is long (i.e., transits are infrequent) and available telescope time is limited. Some terms used to indicate planets of various characteristics, particularly their rough size:

• Earth analogs - roughly the size of Earth, rocky.
• super-Earths - noticeably larger than Earth, often confined to rocky planets.
• mega-Earths - term sometimes used for the largest super-Earths, e.g., more than 10 times the mass of Earth, yet rocky.
• mini-Neptunes - somewhat smaller than Neptune, sometimes confined to ice giants.
• Neptunes - roughly the size of Neptune, gas.
• gas giants - gas planet roughly Saturn size or larger.
• giant planets or extrasolar giant planets (EGPs), similar.
• Jupiters - roughly the size of Jupiter, gas.
• super-Jupiters or super giants - into the brown-dwarf mass range, if considered a planet for other reasons.
• hot Jupiters - Jupiter-like, in tight orbits around the star, e.g., a fraction of a day, thus close to the star and very hot.
• hot Neptunes - similar for Neptune-sized planets.

Terms like super-Neptune are also used to indicate a Neptune-like composition. When a rough classification based purely on mass is used, brown dwarfs would be the next step up from planets, a threshold being bodies of more than 13 Jupiter masses. However sometimes other star- or planet-like characteristics are taken as more important than the mass criteria.

(These terms present a writing style issue: whether Jupiter, Earth, and Neptune should be capitalized, e.g., in hot Jupiter or in Jupiters. I find no rules, and only partial consistency, and have elected to follow the style I find most common.)

The spacecraft Kepler surveyed a portion of the sky for exoplanets, detecting (including K2) 3300 confirmed planets along with 2900 not-yet-eliminated candidates as of 3/2024. More recently, the TESS mission has produced 7500 candidates, including over 400 confirmed planets as of 3/2024. The discovery numbers naturally are affected by bias inherent in the discovery/confirmation techniques, e.g., more of the large and massive planets will be found. Very-rough projected planet demographics including presumed undiscovered planets, based upon the above-listed discoveries with some compensation for such biases:

• Half of star systems have super-Earths.
• 1/6 have Earths.
• 5% have gas giants.
• 1% have hot Jupiters.

The commonly-used designation of exoplanets is somewhat-modeled after the system for individual members of binary stars, but using lower-case letters to indicate the individual planet: the first planet discovered is specified by the star's name followed by "b", the next, "c". For example HD 80606 b is a planet orbiting the star HD 80606. If two or more are discovered simultaneously, they are lettered outward from the innermost.