21 (4) Flashcards
If we can determine the …….. of the star, the transit method tells us the ……… of the planet.
size
The amount of light the planet blocks, called the ………. ………
transit depth
transit depth is given by:
The area of a circle is πR2
R 2
The Doppler method allows us to estimate the …….. of a planet. If the same object can be studied by both the Doppler and transit techniques, we can measure both the ……… and the ……….. of the exoplanet.
mass / mass / size
knowing the mass and the size is a powerful combination that can be used to derive the average ……… (mass/volume) of the planet.
density
Transiting planets reveal such a wealth of information that the French Space Agency (CNES) and the European Space Agency (ESA) launched the …………. space telescope in 2007 to detect transiting exoplanets.
CoRoT
CoRoT discovered 32 transiting exoplanets, including the first transiting planet with a size and density similar to Earth. In 2012, the spacecraft suffered an onboard computer failure, ending the mission. Meanwhile, NASA built a much more powerful transit observatory called ………….
Kepler
(after kepler failure in 2013) A new NASA mission called …………………… is now carrying out a transit survey of the nearer (and therefore brighter) stars all over the sky.
TESS (Transiting Exoplanet Survey Satellite)
What do we mean, exactly, by “discovery” of transiting exoplanets? A single transit shows up as a very slight drop in the brightness of the star, lasting several hours. However, astronomers must be on guard against other factors that might produce a false transit, especially when working at the limit of precision of the telescope. We must wait for a second transit of similar depth. But when another transit is observed, we don’t initially know whether it might be due to another planet in a different orbit. The “discovery” occurs only when a ………… transit is found with similar depth and the same spacing in time as the first pair.
third
the Kepler mission also has a program in which non-astronomers—……….. ……….—can examine the data.
citizen scientists
One of the most convincing ways to verify that a dip in brightness is due to a planet is to find more planets orbiting the same star—a planetary system. Multi-planet systems also provide alternative ways to estimate the …………. of the planets,
masses
………. …….. works best for young gas giant planets that emit infrared light and reside at large separations from their host stars.
Direct imaging
Since 2008 (first three found stars), a number of planets around other stars have been found using direct imaging.
However, one challenge is to tell whether the objects we are seeing are indeed planets or if they are ……….. ………… in orbit around a star.
brown dwarfs (failed stars)
Direct imaging is an important technique for characterizing an exoplanet.
- The brightness of the planet can be measured at different wavelengths. These observations provide an estimate for the …………. of the planet’s atmosphere;
- ……….. can also be obtained from the faint light to analyze the atmospheric constituents, such as hydrogen-rich atmosphere or with evidence of methane
- temperature
- Spectra
the “discovery space” for Kepler was limited to planets with orbital periods less than ……… days and sizes larger than ……….
400 / Mars
The most common planets have radii between 1.4 and 2.8 that of Earth, sizes for which we have no examples in the solar system. These have been nicknamed ……………., while the other large group with sizes between 2.8 and 4 that of Earth are often called ……………
super-Earths / mini-Neptunes.
recall that really small planets were difficult for the Kepler instruments to find. So, to estimate the frequency of Earth-size exoplanets, we need to correct for this sampling bias.
We see that the most common planet sizes of are those with radii from 1 to 3 times that of Earth—what we have called “Earths” and “super-Earths.”
Comparing the average density of exoplanets to the density of planets in our solar system helps us understand whether they are ………… or ………… in nature.
rocky / gaseous
A key observation so far is that planets that are more than 10 times the mass of Earth have ……….. ………. envelopes (like Uranus and Neptune) whereas lower-mass planets are predominately ………… in nature (like the terrestrial planets).
substantial gaseous / rocky
for the highest mass planets (M > 1000 MEarth) in Figure 21.25, notice that the radius stops increasing and the planets with greater mass are actually smaller. This occurs because increasing the mass also increases the …………. of the planet, so that compressible materials (even rock is compressible) will become more tightly packed, shrinking the size of the more massive planet.
gravity
Often, the first step in understanding something new is to ………. …………. the range of possibilities. This sets the stage for refining and deepening our knowledge.
narrow down
Astronomers who work on the complex modeling of the interiors of rocky planets make the simplifying assumption that the planet consists of two or three layers. This is not perfect, but it is a reasonable approximation and another good example of how science works.
Why do some gas giant planets have inflated radii that are larger than the fictitious pure hydrogen planet?
Many of these planets reside in short-period orbits close to the host star where they intercept a significant amount of radiated energy. If this energy is trapped deep in the planet atmosphere, it can cause the planet to expand.
The first planetary system was found around the star ……….. …………. in ……… using the Doppler method,
Upsilon Andromedae / 1999
