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).
2M1207 and its exoplanet 2M1207b from Upright Caesar
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
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
- 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 Huble 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). (Much 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.
The diversity of solar systems - scratching the surface
Exoplanet populations from Wikipedia
- Massive exoplanets are easier to detect by all methods
- Direct imaging (dark red) favors exoplanets with large semimajor axes
- Radial velocity (blue) and especially transit (green) methods favor exoplanets with small semimajor axes.
- Minimum mass
- Semimajor axis
- Orbital period
- Orbital eccentricity
- Density: Using the reduction in brightness in an eclipsing planet as a proxy for volume.
- Albedo (by comparing brightness before and during the planet's passage behind the star.)
- Hints of atmospheric composition and color (determined spectroscopically)
So what have we found?
A hot Jupiter from Softpedia
- Low density. HD 209458 is roughly 0.5 Jupiter masses but with 1.14 times its radius (Snellen et al., 2010).
- They are close enough to their stars to become tidally locked into synchronous rotation. Thus, they ought to have high wind velocities as their atmospheres redistribute heat from their blistering permanent day sides to their cooler permanent night sides. HD 189733 Ab's 5400 mph winds seem to bear this out.
- Hot Jupiters show a wide range of orbital inclinations and eccentricities. Some of them have retrograde orbits and many have orbits that are misaligned with their star's axis of rotation (Queloz et al., 2010). WTF?
View from the surface of COROT-7b
- COROT-7b (right) discovered in 2009, with a semimajor axis of 0.0172 AU and an orbital period of 20 hrs. This is one of the smaller exoplanets , with 4.7 Earth masses. Its surface temperature varies from 2070 to 2870 K. This would cause many substances that exist on Earth as solid minerals to vaporize (Leger et al., 2009).
- If that's not enough, in August 2013 Sanchis-Ojeda et al., 2013 report Kepler 78b, which orbits at 0.01 AU in 8.5 hrs. 1.2 Earth radii and < 8x Earth's mass, it's daytime surface is roughly 5140 K. At this temperature, silica (SiO2), iron, aluminium and other common substances of Earth's crust would exist as gasses, with only the most refractory substances remaining solid. Imagine a world on which a "cold front" would cause droplets of molten iron to condense and rain out of the sky into a globe girdling magma sea.
Gliese581d - super-Earth or mini_Neptune? from Wikipedia
55 Cancri Ae from Wikipedia
- Individual stars in binary systems where the stars are widely separated. E.G.: 55 Cancri Ae (right).
Graziani and Black, 1981, calculate that an exoplanet can maintain a stable orbit around one star of a binary system as long as its apoapsis is less than 20% of the distance of closest approach of the other star. By this standard, 55 Cancri Ae (semimajor axis of 0.0156 AU) is safe, as 55 Cancri B's semimajor axis is 1065 AU.
Kepler 16b from NASA
- Pairs of stars in close binary systems. E.G.: Kepler 16b which orbits a close binary pair. (Estimated temperature)
Possible view from a pulsar planet
TRAPPIST-1b and TRAPPIST-1c transit
TRAPPIST-1 viewed from TRAPPIST-1d from Wikipedia
- escape velocity
- surface temperature.
Every time GEOL212 is taught, there are new record holders. This year's headline makers:
- Kepler438b: At 1.3 Earth-masses, this Earth-like planet orbits at 0.17 AU from a red dwarf star roughly half the sun's mass. This puts it near the inner margin of its Goldilocks zone. With an ESI of 0.88, this is the most Earth-like exoplanet known, though probably not habitable because it it probably locked tidally into a synchronous orbit, with one side in perpetual day and the other in perpetual night.
- The TRAPPIST-1 system: Seven terrestrial planets orbiting a red dwarf star not much larger than Jupiter. Of these, TRAPPIST-1d and TRAPPIST-1e are Goldilocks planets. TRAPPIST-1d (0.3 Earth masses) seems to have a liquid ocean 50 times the mass of Earth's, whereas TRAPPIST-1e (0.77 Earth masses) seems to have a solid surface. TRAPPIST-1d has an ESI of 0.91 - the highest of any exoplanet. (TRAPPIST-1e is 0.86)
Habitable worlds? Don't count on it:
The view from Gliese 667 Cc from Wikipedia
- 1.0 Earth-mass
- ESI of 1.0
- smack in the middle of its star's Goldilocks zone,
- Atmospheric chemistry: Runaway greenhouse effects like we see on Venus could occur on worlds with dense CO2 or water vapor atmospheres, yielding high temperatures.
- Albedo: An exoplanet with a large enough ice cap may experience run-away cooling and ice over. (Earth almost did this between 800 - 600 Ma in the Snowball Earth episode.)
Day side of a synchronously rotating exoplanet orbiting red dwarf
- Orbital characteristics might cause an "Earth-like" exoplanet to be very alien. Especially those in the habitable zones of red dwarfs which are physically much closer to their star than Earth is to the Sun. Consider:
- Rotation: Tidal forces are likely to force a close-in exoplanet to rotate synchronously, with one permanent day side and one permanent night side.
- Tidal heating: And if the orbit is eccentric, significant tidal heating might result in more frequent and extreme volcanism.
- Physical characteristics might be significant:
Core size and rotation: We know nothing of exoplanet magnetospheres. What would an Earth-like exoplanet with a puny or oversized magnetic field be like?
Potential exoplanet volumes from Wikipedia
- Bulk chemistry: There is no reason to think that other solar systems start out with exactly the same materials as ours. Some exotic possibilities:
- Silicate-worlds with thick mantles and tiny cores (like the Moon)
- Iron worlds depleted in silicates (like Mercury)
- Carbon worlds in which carbon-based minerals play the role of silicates. (Imagine a planet where the relative mantle abundances of diamond and olivine are switched!)
- Core size and rotation: We know nothing of exoplanet magnetospheres. What would an Earth-like exoplanet with a puny or oversized magnetic field be like?
The view from Gliese 876 d by Inga Nielsen from Astronomy Picture of the Day
- Red dwarfs: are notorious "flare stars" known to vary greatly in brightness, sometimes breaking out in extensive sunspots and sometimes producing spectacular flares. Alas, the Earth-like Kepler 438b orbits a star like this at 0.166 AU, regularly producing stellar flares ten times greater that the sun's largest. Same goes for Proxima Centauri b (Anglada-Escude et al., 2016), an exoplanet orbiting the closest star to the sun that, at 4.3 light years distance, might conceivably be visited by a space probe. (Artist's impression.)
- Binary systems: Known to support exoplanets, but these must experience variations in illumination during their orbital periods.
Sunset on blue moon from Softpedia
How would an Earth-mass blue moon in the middle of a star's Goldilocks zone differ from Earth?
The early Earth
- Photosynthesizers were cranking out oxygen here for a billion years before significant quantities accumulated in the atmosphere about 2 ga.
- Multicellular organisms only appeared in the oceans about 1 ga.
- The land was utterly lacking in multicellular life until about 450 ma.
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
- Exoplanet diversity:
- Hot Jupiters
- Cthonian planets
- Pulsar planets
- Super-Earths and Mini-Neptunes
- Goldilocks planets
- Earth-like ≠ habitable
- Blue moons
- Guillem Anglada-Escudé, Pedro J. Amado, John Barnes, Zaira M. Berdinas, R. Paul Butler, Gavin A. L. Coleman, Ignacio de la Cueva, Stefan Dreizler, Michael Endl, Benjamin Giesers, Sandra V. Jeffers, James S. Jenkins, Hugh R. A. Jones, Marcin Kiraga, Martin Kürster, Maria J. Lopez-Gonzalez, Christopher J. Marvin, Nicolas Morales, Julien Morin, Richard P. Nelson, José L. Ortiz, Aviv Ofir, Sijme-Jan Paardekooper, Ansgar Reiners, Eloy Rodríguez et al.. 2016. A terrestrial planet candidate in a temperate orbit around Proxima Centauri. Nature 536, 43-440. doi:10.1038/nature19106.
- 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).
- Julien de Wit, Hannah R. Wakeford, Michael Gillon, Nikole K. Lewis, Jeff A. Valenti, Brice-Olivier Demory, Adam J. Burgasser, Laetitia Delrez, Emmanuel Jehin, Susan M. Lederer, Amaury H. M. J. Triaud, Valerie Van Grootel. 2016. A combined transmission spectrum of the Earth-sized exoplanets TRAPPIST-1 b and c. Nature July 2016 preprint.
- 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).
- Michael Gillon, Emmanuel Jehin, Susan M. Lederer, Laetitia Delrez, Julien de Wit, Artem Burdanov, Valerie Van Grootel, Adam J. Burgasser, Amaury H. M. J. Triaud, Cyrielle Opitom, Brice-Olivier Demory, Devendra K. Sahu, Daniella Bardalez Gagliuffi, Pierre Magain, and Didier Queloz. 2016. Temperate Earth-sized planets transiting a nearby ultracool dwarf star. Nature 533, 221-224
- Rene Heller, Michael Hippke, Ben Placek, Daniel Angerhausen and Eric Agol. 2016. Predictable patterns in planetary transit timing variations and transit duration variations due to exomoons. Astronomy and Astrophysics 591.
- A. Leger, D. Rouan, J. Schneider, P. Barge, M. Fridlund, B. Samuel, M. Ollivier, E. Guenther, M. Deleuil, H. J. Deeg, M. Auvergne, R. Alonso, et al.. 2009. Transiting exoplanets from the CoRoT space mission: VIII. CoRoT-7b: the first super-Earth with measured radius. Astronomy and Astrophysics 506 (1) 287-302.
- Roberto Sanchis-Ojeda, Saul Rappaport, Joshua N. Winn, Alan Levine, Michael C. Kotson, David W. Latham, and Lars A. Buchhave. 2013. Transits and occultations of an Earth-sized planet in an 8.5 hr orbit. The Astrophysical Journal 744(1).