Astronomy Week 14

Why did it take astronomers until 1995 to discover the first exoplanet orbiting another star like the Sun?

Astronomers had to build spectrometers sensitive enough to detect the very small Doppler shifts in the star's spectral pattern.

The big surprise about the first planet discovered around another regular star was that it _____.

orbited so close to its star it took only 4 days to go around

The telescope that allowed astronomers to discover most of the planets found with the transit method was called _____.

the Kepler mission

Why is the existence of "hot Jupiters" puzzling? What is the current hypothesis for how they formed?

"Hot Jupiters" are found near stars in very hot places where they should not have formed and show evidence of boiling away. These planets migrated recently from farther out to their current close distance.

(a) How do you think the age of another planetary system would compare to the age of our solar system? (b) What is the reason for the answer to the previous question?

(a) It should be similar; It should be older; It should be younger... It could be any of the above. (b) Our solar system is about halfway through its evolution, and our sun is probably a second- or third-generation star.

(a) According the solar nebula hypothesis, which of the following is true? (b) What is the reason for the answer to the previous question? (c) Is there any evidence to support the answers to the previous questions?

(a) Planetary systems should be common. (b) According to the theory, planets form from the gases surrounding a star after it forms. (c) Yes; disks of gas and dust and actual planets have been discovered orbiting other stars.

When astronomers carefully examine the planets found by Kepler and draw conclusions from the Kepler sample, what do they conclude about planets the size of Earth?

Earth-sized planets are common, but so are planets somewhat bigger than Earth

Our solar system has 8 planets orbiting the Sun. Based on the discoveries of exoplanets so far, what can we say about a star with 8 planets?

Even though planets were only discovered starting in 1995, so we can't yet find planets that take centuries to go around their star, we already know other stars with 8 planets and we are likely to find more

List three ways in which the exoplanets we have detected have been found to be different from planets in our solar system. (Select all that apply.)

Gas giant exoplanets can reside close to their star; Some exoplanets orbit binary stars; Small rocky exoplanets commonly have orbits smaller than Mercury's.

Which of the following is a reason that astronomers have not found giant planets with the orbit of Neptune around other stars?

Neptune takes 165 years to go around the Sun; getting information about just one cycle of such a planet's orbit around another star would take astronomers 165 years

Why haven't we detected low-mass planets close to their stars and high-mass planets far from their stars?

Our techniques are not yet sensitive enough and we have not been observing for a long enough time.

How do we know that extrasolar planets are orbiting other stars?

We see a star's light dim as a planet passes in front of the star and detect alternating Doppler shifts in the spectra of some stars.

What revisions to the theory of planet formation have astronomers had to make as a result of the discovery of exoplanets? (Select all that apply.)

Planet formation is more chaotic and less orderly than we imagined; Planets can migrate in the protoplanetary disk through gravitational friction or drag; It is possible to have stable planets orbiting a system of two stars.

Why was the Kepler mission not able to find planets smaller than Mars, even though it was in space (and had no Earth atmosphere to deal with)?

Such planets make dips in the light of the star that are too small for Kepler to detect

Why is it so hard to see planets around other stars and so easy to see them around our own? (Select all that apply.)

The light from the planets is very faint compared to the overwhelming glare of the host star; We can easily see the planets around our solar system because they reflect the light from the Sun.

Why were giant planets close to their stars the first ones to be discovered? (Select all that apply.) Why has the same technique not been used yet to discover giant planets at the distance of Saturn?

Their short periods allow for detection using the radial velocity technique; They are massive enough to impose a detectable gravitational tug on their stars. The planet would have to be observed for a very long time to be detected.

What are the general characteristics of the extrasolar planets discovered so far?

They have high mass and orbit close to their stars.

What evidence can you cite that planets orbit other stars?

disks of gas and dust around stars; doppler shifts in the spectra of sun-like stars; a decrease in the brightness of a star when a planet crosses in front of it

What technique did astronomers use to make the first confirmed discovery of a planet around another star like the Sun?

measure the Doppler shift of the lines in the star's spectrum and look for periodic changes in this shift due to the pull of the planet as it orbits the star

Astronomers were surprised to find so many Jupiter-mass planets so close to their stars. According to their best theories and models, such "hot Jupiters" _____.

must have formed further out from the star and must have "migrated inward" early on

Which of the following is NOT one of the reasons planets around other stars are so difficult to detect?

planets only form very late in the life of a star, just when it is ready to die, and thus last only a very short fraction of the star's life

Which types of planets are most easily detected by Doppler measurements? (Select all that apply.) Which types of planets are most easily detected by transits? (Select all that apply.)

such planets should orbit close to their star, such planets should be high in mass such planets should orbit close to their star, such planets should have large radii

With our current techniques, astronomers can typically only measure the minimum mass of a planet orbiting another star. To know the precise mass of the planet, they must also be able to determine _____.

the angle at which the planet's orbit is tilted relative to us

What observations about disks of dusty material around young stars suggest that planets may be forming in such disks?

the disks show lanes that are empty of dust within them

To measure how dense a planet is (to know whether it is made of rock or gas and liquid) they must be able to measure the planet's mass from the Doppler shift and _____.

the planet's radius using the transit method