Exoplanets - How do we know anything about other solar systems?

Extrasolar planet or Exoplanet - Any object that would fit the IAU definition of a planet but for the fact that it circles a star other than the Sun. This typically excludes Brown dwarves: Objects from 13 to 80 Jupiter masses that have undifferentiated convective interiors, do not experience sustained fusion of hydrogen, but may sustain brief episodes of fusion of deuterium (2H).

Exoplanet history

2M1207 and its exoplanet 2M1207b from Upright Caesar
The first reliably confirmed exoplanet claim came in 1988, when Bruce Campbell, G. A. H. Walker, and S. Yang identified an exoplanet circling γ Cephei A (44.9 ly) (Hatzes et al., 2003). The primary detection methods are indirect, so we have direct visual observations of 10 exoplanets, including Fomalhaut b (25 ly) and 2M1207b (right 230 ly) the first to be seen in 2005.

As of June, 2020, there are 4164 confirmed exoplanets.

Direct imaging

HR8799 from Wikipedia
The image at right shows HR8799 and its three planets - the first solar system to be observed directly. Direct methods imaging of exoplanets are highly problematic because planets are extremely dim compared to the stars they orbit. Observations have been made by coronagraphs, instruments that block the light of a bright object (originally developed for viewing the Sun's corona) and allow viewing of faint objects near them. Even so, the only planets that can be seen are:

For more images, link to Phil Plait's gallery of directly imaged exoplanets.

Indirect identification

In contrast, indirect methods of exoplanet detection are proving very effective. As of November, 2018, 3,874 confirmed exoplanets have been found indirectly, and many more candidate worlds are known.

The Alpha Centauri system from Universe Today
Exoplanet nomenclature: Exoplanets are designated by affixing a lower-case letter to the name of their star. Successive discoveries get sequential letters. E.G.: Fomalhaut b is the first planet to be discovered orbiting Fomalhaut. If another exoplanet were to be discovered there, it would be called Fomalhaut c. (The sequence of letters indicates nothing about the exoplanet's mass or semimajor axis. "Fomalhaut a" is the star, itself.)

Note: Members of multiple star systems are designated with upper-case letters in order of descending mass. Thus, the Alpha Centauri system contains α Centauri A and α Centauri B, but α Centauri A is slightly larger. When exoplanets are involved, we combine the two systems. E.G.: α Centauri Ab would be an exoplanet circling α Centauri As.

What if an exoplanet is found to orbit both Alpha Centauri A and B? It would be Alpha Centauri (AB) b.

Methods of indirect identification

Radial velocity: As an exoplanet orbits a star, the star moves in a small orbit around the system's barycenter or common center of gravity. This results in variations in the speed with which the star moves toward or away from an observer on Earth, causing displacement - red or blue shifting - of absorption lines in its spectrum. These displacements can be measured with great precision (down to 1 m/s), allowing the star's orbit around the barycenter to be calculated. From this the orbital properties and mass of the orbiting planet can be inferred.

The transit method from The TEP Network
Transit method: If the plane of an exoplanet's orbit is aligned with Earth, then when the exoplanet crosses in front of the star, eclipsing it, there will be s slight drop in the star's observed brightness. The amount of dimming depends on the star's and exoplanet's relative size. After the radial velocity method, this has been the most productive. This method has pros and cons:

The transit method from the Hubble Space Telescope page
Consider HD 189733 Ab, a "hot Jupiter" (roughly 0.8 Jupiter masses) orbiting an orange dwarf star at a sizzling distance of 0.032 AU (Berdyugina et al., 2011):

The Kepler Orbital Observatory coverage area
The Kepler Mission: From 2009 to 2013, the Kepler spacecraft scanned stars in a section of the sky with unprecedented sensitivity, searching for transiting planets. Kepler has detected transiting Earth-sized exoplanets. As of July 2016 it had discovered 2326 confirmed by additional observations.

Note: Both of these methods have observational biases, favoring the discovery of exoplanets that are:

Additional exoplanet identification methods:

Transit timing variation method (TTV): If an exoplanet has been identified using the transit method, and sufficient transits have been observed to characterize its orbital period, then additional exoplanets can be identified by variations in the regularity of transits, as these are cause by perturbations of the exoplanet's orbit by the gravity of other exoplanets.

Gravitational microlensing: Gravitational lensing occurs when the gravity of a massive object focuses light coming form an object behind it. This is frequently observed in galaxies but can also be caused when two stars are aligned along a line of sight from Earth (microlensing). In this case, because the alignment must be perfect and Earth and the stars are moving relative to one another, microlensing events are brief (days or weeks).

The impressive part is that if the lensing star has an exoplanet in orbit, the exoplanet's gravity can have a detectable effect on the lensing. Thirteen exoplanets have been identified this way. Pros and cons:

Key concepts and vocabulary:
Additional reading: