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 around a main sequence star (51 Pegasi) in 1995. As of 12/2019, 4100+ planets around 3000+ 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 (transiting planet, the transit method).
• direct imaging: seeing the planet.
• transit timing variations, i.e., after a planet's transits are seen, 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: watching the star's motion.
• pulsar timing: pulses vary due to orbiting planet.
• circumstellar disks: dust near a star suggests a cause, e.g., collisions.
• circumstellar disks: a planet can create a detectable gap in a disk.
• circumstellar disks: planet gravity can create a clump in a disk.

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, to 10-or-so Earth masses, sometimes confined to rocky planets.
• mega-Earths - 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.
• hot Neptunes - similar for Neptune-sized planets.

Terms like super Neptune are also used to indicate a Neptune-like composition. Using a criteria based on mass, brown dwarfs are the next step up from planets, for objects 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) 5500 not-yet-eliminated candidates, of which 2700 have been confirmed to be planets as of 12/2019.

Projected population statistics based upon surveys:

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

The commonly used method of referring to exoplanets is somewhat-modeled after the system for individual members of binary stars. 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.