Nov. 20th - More Exoplanets Flashcards
Atmospheric Composition & Temperature:
Atmospheric Composition
- Direct observations have already provided some data, and are expected to provide much more with the aid of new instruments on ground-based telescopes that can obtain images and spectra of exoplanets at the same time.
- We can also learn about atmospheric composition from transiting systems by comparing a system’s spectrum taken with the planet in front of its star (the transit) or behind it (the eclipse) to spectra taken at other times
Temperature: Transit Systems
For transiting systems, key data come from eclipses.
* Recall that planets generally emit infrared light, and the amount of infrared emission (per unit area) depends on a planet’s temperature.
* As a planet goes behind its star (the eclipse), the system’s infrared brightness will drop because we are no longer seeing the planet’s infrared emission.
* The extent of the drop tells us how much infrared the planet emits, and we can combine knowledge of the amount of infrared emission with the planet’s radius (measured by the transits) to calculate an approximate temperature.
Temperature: Transit Systems CONT.
- Transiting systems can sometimes allow us to learn even more about temperature by monitoring changes in a system’s visible-light brightness as a planet presents different phases to Earth.
- That is, much as we see phases of Venus from Earth, we see a planet as “new” during a transit and as nearly “full” just before and after an eclipse.
- With infrared light, we can observe differences in brightness over a planet’s day and night sides as it orbits. Moreover, all planets on close-in orbits are expected to have synchronous rotation so that they show the same face to their star at all times, just as the Moon always shows the same face to Earth
- Therefore, we can in principle use infrared observations to create a crude “weather map” of an extrasolar planet.
How do exoplanets compare to planets in our solar system?
Orbital Properties
- A few exoplanets have been found with orbits similar to those of planets in our solar system, but many others exhibit orbital properties that seemed quite surprising when they were first discovered.
- In particular, many planets with Jupiter-like masses or sizes orbit quite close to their stars—in many cases much closer than Mercury orbits the Sun—which is surprising because the jovian planets in our solar system all orbit quite far from the Sun.
- One other interesting surprise has come from binary star systems: Scientists had not been sure whether planets could form and have stable orbits in binary star systems, but data from the Kepler mission have shown that they can: means that some worlds have two (or more) “suns” in the sky, much like the fictional planet Tatooine
Sizes, Masses, and Densities - The Kepler mission detected many exoplanets
Two remarkable conclusions are already apparent:
First, planets are common:
* By looking across all size categories, astronomers conclude that at least 70% of all stars harbor at least one planet
Second, many of these planets are quite small, suggesting that Earth-size planets are very common
* Again, keep in mind that these statistics remain incomplete: Because it takes longer to identify planets with larger orbits, the current statistics reflect only planets with relatively short orbital periods
* Given that we expect most jovian planets to have long orbital periods, it’s likely that there are many more high-mass planets than the current data suggest. The current data may also underestimate the number of small planets, because these planets are so difficult to detect.
The Nature of Exoplanets
Exoplanets: Do they fall into the same terrestrial and jovian categories as the planets in our solar system, or do we find additional types of planets?
We cannot yet know for certain, because we do not yet have direct ways of obtaining spectra with sufficient detail to determine the basic compositions of exoplanets.
Nevertheless, because we have measured the abundances of different chemical elements among other stars, we know that all planetary systems must start out from gas clouds generally similar in composition to the solar nebula. That is, all star systems are born from gas clouds containing at least about 98% hydrogen and helium, sprinkled with much smaller amounts of ice, rock, and metal
The Nature of Exoplanets
Exoplanets: Terrestrial similarities?
Super-Earths
We also see many planets that appear to be terrestrial in nature, with compositions of rock and metal.
For example, COROT-7b has an average density near 5 g/cm3, comparable to Earth’s density. This density suggests that it has an Earth-like composition, so the fact that it has a mass about five times that of Earth makes COROT-7b an example of what is sometimes called a: “super-Earth”. It orbits very close to its star, so its surface is probably molten.
The Nature of Exoplanets
Exoplanets: non-Terrestrial, non-Jovian?
Super-Earths, Mini-Neptunes
- Some planets aren’t clearly in either the terrestrial or the jovian category.
- In fact, planets with masses between those of Earth and Neptune—often called either super-Earths or mini-Neptunes—are the most common type of planet discovered so far.
- Because we don’t have a planet in this mass range in our own solar system—and because this mass range falls in between that of our solar system’s terrestrial and jovian worlds—we don’t have a clear idea of what their compositions might be
- Some on the smaller end of this mass range probably have a rock/metal composition similar to that of the terrestrial worlds in our solar system
The Nature of Exoplanets
Exoplanets: Water-dominant
- Others (perhaps GJ 1214b) appear to fit a model suggesting that they are made predominantly of water (perhaps in unusual phases) or other hydrogen compounds.
- Planets of this type are often called “water worlds”, though some scientists prefer to use only the more generic term “mini-Neptunes.
Exoplanet Challenges to the Nebular Theory
Exoplanet discoveries present at least two challenges that are not immediately explained by the nebular theory as applied only to our solar system
- Many exoplanets do not fall neatly into either the terrestrial or the jovian categories that seem to fit all the planets in our solar system.
- Many Jupiter-size exoplanets have been found to have close-in or eccentric orbits.
Recall that, according to the nebular theory, jovian planets form as gravity pulls in gas around large, icy planetesimals that accrete in a spinning disk of material around a young star.
The theory therefore predicts that jovian planets should form only in the cold outer regions of star systems (because it must be cold for ice to condense), and that these planets should be born with nearly circular orbits (matching the orderly, circular motion of the spinning disk).
Exoplanet Challenges to the Nebular Theory
Which surprising orbits of exoplanets caused scientists to reexamine the nebular theory of solar system formation?
Mention of MIGRATION
Planets were massive and had close-in orbits, making scientists wonder whether something might be fundamentally wrong with the nebular theory: it now seems much more likely that its basic outline is correct
Scientists therefore suspect that jovian-type exoplanets were indeed born with circular orbits far from their stars, and that those that now have close-in or eccentric orbits underwent some sort of “planetary migration.”
Planetary Migration - Our Solar System
The law of conservation of energy demands that Jupiter must have migrated inward, losing the same amount of orbital energy that the comets gained.
Jupiter’s inward migration may, in turn, have caused outward migration for Saturn, Uranus, and Neptune that, in turn, shaped the orbits of objects in the Kuiper belt
Planetary Migration - OTHER Planetary Systems
- Much greater migration may occur in other planetary systems, particularly if the planets lose energy to gas before the gaseous disk is cleared out.
- Models suggest that this may occur because a planet’s gravity creates waves that propagate through the disk, causing material to bunch up as the waves pass by
- This “bunched up” matter (in the wave peaks) then exerts a gravitational pull on the planet that tends to reduce its orbital energy, causing the planet to migrate inward toward its star
Planetary Migration - OTHER Planetary Systems: “Swallowed Planets”
- In some cases, the planets may form so early that they end up spiraling all the way into their stars
- Assortment of elements in their outer layers, suggesting that they may have **swallowed planets **(including migrating jovian planets and possibly terrestrial planets shepherded inward along with the jovian planets).
- These ideas are not just hypothetical: Several hot Jupiters orbit so close to their stars that it seems likely that tidal forces will send them on death spirals into their stars on time scales of just millions of years.
How can planetary migration explain orbital eccentricities of exoplanets?
Related mechanisms may explain the surprisingly high orbital eccentricities of many exoplanets.
For example, planetary migration* increases the chances that planets will influence each other gravitationally*. In some cases, planets may pass close enough for a gravitational encounter in which one planet gains enough energy to escape from the star system entirely (becoming an “orphan planet”) while the other is flung inward into a highly elliptical orbit.
In other cases, continuing gravitational tugs may lead to orbital resonances much like those that cause the orbits of Jupiter’s moons Io, Europa, and Ganymede to be more elliptical than they would be otherwise
