Exoplanets - What do we really know about solar systems?

Extrasolar planet or Exoplanet - Any object that would fit the IAU definition of an exoplanet but for the fact that it circles a star other than the Sun. The term "planet" typically excludes Brown dwarfs: 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

Since the recognition that the Sun is the center or the Solar System, scholars, including Giordano Bruno and Isaac Newton have speculated that other stars might also harbor planets. The first systematic attempt actually to identify them were:

Both sets of claims were retracted after closer scrutiny.

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 (44.9 ly). Recent news items indicate that we just past the 1000 confirmed exoplanet mark, with many coming from multiple-planet systems. 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.

But in the news of Aug. 2013: A new exoplanet has been imaged: The smallest ever around a sun-like star - GJ 504. A 4.5 Jupiter-mass jovian planet orbiting GJ 504 at roughly 43.5 AU. GJ 504b, being only 160 million years old, glows brightly in the infrared.

Direct imaging

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:

Q: Why might 2M1207b have been easier to spot?

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. Over 1000 confirmed exoplanets have been found indirectly, and over 3000 candidate worlds are known. Indirect methods are discussed below. Indeed, the 2012 XKCD cartoon at right is now dated.

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 Bb would be an exoplanet circling α Centauri B, the second most massive of the three star α Centauri system. When these notes were first composed, this was a contrived hypothetical. During Oct. 2012, however, the real α Centauri Bb was identified. Roughly Earth sized but orbiting every 3.2 days at a blistering distance of 0.04 AU.

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.

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:


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:


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 2013 it had discovered 3277 candidate exoplanets with 134 confirmed by additional observations. The smallest, Kepler 37b is slightly larger than the moon. (Curb your enthusiasm. It's semimajor axis is 0.1003 AU and it's surface roasts at 700 K.)

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


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. This is essentially the application of the method Le Verrier used to identify Neptune to other solar systems.

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:


Pulsar timing: A pulsar is type of neutron star, a super-dense remnant of a star that has exhausted its nuclear fuel. These objects compress not just the mass but the magnetic energy of the original star into a very small volume. These powerful magnetic fields focus radio emissions along field lines. Called pulsar because the rotation of these focused radio emissions as the pulsar rotates appear to an observer on Earth as regular radio wave pulses. Gravitational interactions between pulsars and planets orbiting them result in perturbations of the period of the pulses that can be detected and interpreted. Yes, Virginia, there are exoplanets circling pulsars.

The diversity of solar systems - scratching the surface

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?

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:


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:


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. This gap is being filled by exoplanets of intermediate mass. Depending on one's mood, these are called:

A given exoplanet's characteristics, including the presence of a distinct surface, would depend on composition and temperature as well as mass. Generally, unless they are quite hot, they would retain hydrogen in their atmospheres. At what point, exactly, their distinct surfaces give way to supercritical fluids of increasing density also depends on temperature and composition.

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:


Pulsar planets: Planets have been discovered circling stellar remnants. Pulsar PSR B1257+12, for instance, is circled by at least four, which range in size from 1.5 Earth masses to something in the dwarf planet range - the first ever 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.

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 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:


Habitable worlds? Don't count on it:

Earth-like ≠ habitable: Before you get too excited, even if we found a world of 1.0 Earth-mass smack in the middle of its star's Goldilocks zone, it might still be a very alien place. (Even Venus and Mars are technically inside the Sun's Goldilocks zone but uninhabitable owing to atmospheric characteristics.) Thus peculiarities of the following attributes might make a planet inhospitable to liquid water:


Stellar characteristics: Not all stars are polite.

Above right, an artist's impression of the super-Earth Gliese 667 Cc, orbiting a red dwarf in a three-star system.

Blue moons: We currently have no way of detecting exoplanets' moons. That's a shame considering that 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?

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: