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 and the first around a main sequence star (51 Pegasi) in 1995. As of 2/2019, 3900+ planets around 2800+ 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, 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.
• a planet can create a detectable gap in a disk.
• 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 field affecting the star's field.

The RV method reveals the relative density 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 get both if that possibility looks promising, and leads 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:

• Earths - 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 - noticeably smaller than Neptune, sometimes confined to gas planets.
• Neptunes - roughly the size of Neptune, gas.
• gas giants - gas planet roughly Saturn size or larger.
• Jupiters - roughly the size of Jupiter, gas.
• super Jupiters or super giants - up to 80-or-so Jupiter masses.
• hot Jupiters - Jupiter-like, in tight orbits around the star, e.g., a fraction of a day.
• hot Neptunes - similar for Neptune-sized planets.

Larger than super Jupiters are brown dwarves. Terms like super Neptune are also used to indicate a Neptune-like composition.

(These terms present a 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 2/2019.

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.