Exoplanets - What do we really know about 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). We now know of roughly 3500 confirmed exoplanets. 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.

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 Huble 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.

From Wikipedia
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 form 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:

The diversity of solar systems - scratching the surface

Exoplanet populations from Wikipedia
Generalizations: A plot of masses and semimajor axes of known exoplanets reveal predictable observational biases: Note: the low-mass outlier orbits a pulsar. Typically, the only things we know about these worlds are: Under favorable conditions, we can sometimes determine:

So what have we found?

A hot Jupiter from Softpedia
Hot Jupiters: Observational bias or not, the existence of so many massive planets orbiting very close to their stars was a great surprise, as our models of solar system formation do not allow them to form there. Compounding the surprise:

View from the surface of COROT-7b
Cthonian planets: Both theoretical models and observations suggest that some hot Jupiters are losing their atmospheres, either because stellar winds are stripping them away or because gas is being sucked away by the tidal forces of the star's gravity. What is left when the gas is gone? the exoplanet's naked rocky/metallic core. Examples:

Gliese581d - super-Earth or mini_Neptune? from Wikipedia
Super Earths and mini-Neptunes: In our solar system a huge gap separates Earth, the largest terrestrial planet, from Uranus (14 Earth-masses), the smallest Jovian planet. In other systems, planets of this size range appear to be among the most common. Depending on one's mood, these are called: Whether these would resemble a large Earth (or Venus), with a solid surface; or a small Neptune, with a massive hydrogen atmosphere, would depend on the specifics of its mass, composition, orbit, temperature, and interactions with stellar wind.

55 Cancri Ae from Wikipedia
Binary star planets: The degree to which exoplanets can occupy stable orbits in multiple star systems is a topic of debate. Recent discoveries have confirmed that exoplanets can orbit both:

Possible view from a pulsar planet
Pulsar planets: Konacki and Wolszczan, 2003 report exoplanets circling the stellar remnant - pulsar PSR B1257+12. There are at least four, which range in size from 1.5 Earth masses to PSR B1257+12b, with roughly two lunar masses - the first known dwarf exoplanet. The bizarre thing is that the stellar cataclysm that left PSR B1257+12 as a remnant should have destroyed any exoplanets circling the original star. Where did the exoplanets we see come from? In principle, an exoplanet might actually survive while orbiting inside a star's photosphere during its red giant stage, but it seems more likely that these planets accreted from the planetary nebula thrown off by the star's death.

TRAPPIST-1b and TRAPPIST-1c transit
TRAPPIST-1 viewed from TRAPPIST-1d from Wikipedia
Golidlocks planets: Exoplanets orbiting within a star's Goldilocks zone - the region in which water can exist as a liquid on an exoplanet's surface. Such worlds might harbor life or even be potentially habitable to humans, if we could ever get there. So far, a truly Earth-like exoplanet has not been found, but we are getting closer. Goldilocks planets are rated on an Earth Similarity Index (ESI) that considers: on a scale of 0.0 to 1.0 (Earth.) (Note: Venus' ESI is 0.444. Mars' is 0.797.)

Every time GEOL212 is taught, there are new record holders. This year's headline makers:

Habitable worlds? Don't count on it:

The view from Gliese 667 Cc from Wikipedia
Earth-like ≠ habitable: Before you get too excited, even if we found a world with: it might still be a very alien place. Thus peculiarities of the following attributes might make a planet inhospitable to liquid water:

The view from Gliese 876 d by Inga Nielsen from Astronomy Picture of the Day
Stellar characteristics: Not all stars are polite.

Sunset on blue moon from Softpedia
Blue moons: We currently have no operational way of detecting exoplanets' moons, although concepts are being explored. (See Heller et al., 2016.) Let's hope they work it out, because some jovian exoplanets like ε Andromedae d appear to orbit within their star's Goldilocks zone. At 1.28 Jupiter masses, it probably does not harbor an Earth-like moon, but might (from the rule of 1/10,000) have Mars-sized moon. Larger Jovian exoplanets might be orbited by Earth-like moons.

How would an Earth-mass blue moon in the middle of a star's Goldilocks zone differ from Earth?

The early Earth
Life: Even Earth, itself, would have seemed alien and inhospitable for 4/5 of its history because of the absence of a substantial biosphere: To be habitable, an Earthlike world would need a substantial biosphere, preferably made of creatures based on similar biochemistry. (Cf. "Pandora.")

Key concepts and vocabulary:
Additional reading: