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).
2M1207 and its exoplanet 2M1207b from Upright Caesar
As of June, 2020, there are 4164 confirmed exoplanets.
HR8799 from Wikipedia
- Very large (roughly Jupiter mass or larger)
- Orbiting far from their primary
- Very bright in infrared
For more images, link to Phil Plait's gallery of directly imaged exoplanets.
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
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
The transit method from The TEP Network
- Cons: Only planets whose orbits align with Earth can be discovered. This is a small proportion of suspected planets.
- Pros: When the exoplanet eclipses its star, starlight passes through the exoplanet's atmosphere. By subtracting the star's base-line spectrum from the composite that is formed during transit, we can study the composition of the exoplanet's atmosphere spectroscopically.
The transit method from Phil Plait's Bad Astronomy 8/20/2013
- Also, during secondary eclipse when the star eclipses the exoplanet, we can compare the combined brightness of the star and exoplanet (before the eclipse) with the brightness of the star alone during eclipse, allowing us to assess the exoplanet's albedo. (Right: brightness of Kepler78 showing primary and secondary eclipses.)
The transit method from the Hubble Space Telescope page
- By breaking down the exoplanet's brightness during secondary eclipse by region of the spectrum, it has been found to be much brighter in blue light, suggesting that it has a deep blue color (Evans et al., 2013.)
- Comparisons of changes in the star's brightness during exoplanetary transit in visible light and X-rays has enabled us to assess the thickness of its atmosphere (Poppenhaeger et al., 2013). (About 30% of the atmosphere is transparent to visible light but opaque to X-rays.)
- The doppler shift of absorption lines of sodium in the exoplanet's atmosphere has been used to measure its wind speeds, with max out at 2 km/sec (~5400 mph) (Louden and Wheatley, 2015).
The Kepler Orbital Observatory coverage area
Note: Both of these methods have observational biases, favoring the discovery of exoplanets that are:
- orbiting very close to their parent star (the opposite of direct imaging)
- orbit relatively small stars (so that the effect of the exoplanet is proportionally large)
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.
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:
- Cons: Detection of an exoplanet by microlensing is a one-shot opportunity. Follow up using other methods or repeated observations are impossible. Also, because one must opportunistically exploit any alignment of stars, there is no way to limit the search to nearby star systems where follow up might be possible.
Key concepts and vocabulary:
- Brown dwarf
- Exoplanet naming convention
- Direct imaging
- Indirect detection:
- Radial velocity method
- Transit method
- Primary and secondary eclipse
- Transit timing variation method
- Gravitational microlensing
- Pulsar timing
- S. V. Berdyugina, A. V. Berdyugin, D. M. Fluri, and V. Piirola. 2011. Polarized reflected light from the exoplanet HD189733b: First multicolor observations and confirmation of detection. The Astrophysical Journal Letters 728(1).
- Thomas M. Evans, Frederic Pont, David K. Sing, Suzanne Aigrain, Joanna K. Barstow, Jean-Michel Desert, Neale Gibson, Kevin Heng, Heather A. Knutson, and Alain Lecavelier des Etangs. 2013. The deep blue color of HD189733b: Albedo measurements with Hubble Space Telescope/ space telescope imaging spectrograph at visible wavelengths. The Astrophysical Journal Letters 772(2).