Astronomy-101

When is the soonest we are likely to have moderate-resolution images and spectra of Earthlike planets around other stars?

Any day now, thanks to our largest ground-based telescopes.

Suppose you are using the Doppler technique to look for planets around another star. What must you do?

Compare many spectra of the star taken over a period of many months or years.

Which of the following will allow you to learn something about a transiting planet's atmospheric composition?

Compare spectra obtained before and during an eclipse.

Each item describes a characteristic that applies to one of the three planet-detection techniques shown following. Match the items to the correct planet-finding technique.

Doppler technique- used for most of the first 200 extrasolar planet detections currently best suited to find Jupiter sized extrasolar planets orbiting close to their stars Transit technique-planet detection strategy of NASA's Kepler mission looks for very, slight, periodic dimming of a star allows for the extrasolar planet's radius to be determined offers the best chance of finding Earth-sized extrasolar planets in the next few years can potentially detect planets in only a few percent of all planetary systems ASTROMETRIC TECHNIQUE- measures precise changes in a star's position in the sky, in fractions of arcesconds

The following images show the set of four stars and planets as in Part B. In reality, the center of mass would be inside the star, but for the purposes of clarity in this problem, we're showing it at distinctly different distances from the center of the star. Imagine that an Earth-based observer could see the motion of each of the stars edge-on. Rank each star based on the amount of Doppler shift we'd see in its spectrum as it moves in response to the gravitational tug of its planet, from smallest to largest. (Not to scale)

From smallest doppler shift to largest doppler shift 2.5AU, 2AU, 1.5AU, 1AU

From the viewpoint of an alien astronomer, how does Jupiter affect observations of our Sun?

It causes the Sun to move in a small ellipse with an orbital period of about 12 years.

Based on everything you have learned about the formation of our solar system, which of the following statements is probably not true?

Only a tiny percentage of stars are surrounded by spinning disks of gas during their formation.

Assuming that our ideas about how "hot Jupiters" ended up on their current orbits are correct, why didn't our own solar system end up with any hot Jupiters?

Our solar nebula must have been blown into space shortly after the formation of the jovian planets.

The following figures show four identical Sun-like stars and their companion planets. In reality, the center of mass would be inside the star, but for the purposes of clarity in this problem, we're showing it at distinctly different distances from the center of the star. Each planet's orbital distance is given in Astronomical Units (AU); note that in this case, all four planets have the same mass. Rank the extrasolar planets based on the amount of time it takes each to complete one orbit, from shortest to longest. (Not to scale)

Shortest to longest 1AU, 1.5AU, 2 AU, 2.5 AU

You observe a star very similar to our own Sun in size and mass. This star moves very slightly back and forth in the sky once every 4 months, and you attribute this motion to the effect of an orbiting planet. What can you conclude about the orbiting planet?

The planet must be closer to the star than Earth is to the Sun.

Consider the planet that causes the stellar motion shown in Plot 2 (be sure you have clicked the "Plot 2" button in the lower window of the animation). What can be said about a different planet orbiting the same star with an orbital period of 500 days?

The planet must be closer to the star.

Which of the following properties can be inferred from the star's orbital period?

The planet's orbital radius

Why is it so difficult to take pictures of extrasolar planets?

Their light is overwhelmed by the light from their star.

Current evidence suggests that some massive jovian planets orbit at very close orbital distances to their stars. How do we think these planets ended up on these close orbits?

These planets migrated inward after being born on orbits much farther from their stars.

To date, we've found very few planets orbiting their stars at distances comparable to the distances of the jovian planets in our solar system. Why do astronomers think this is the case?

We have not yet been searching for planets at such distances for a long enough time.

The following images show four identical Sun-like stars and their companion planets, all traveling in circular orbits. In each case, the mass of the planet is given in Jupiter masses and the orbital distance is given in Astronomical Units (AU). Rank each case based on the strength of the gravitational force exerted by the extrasolar planet on its central star, from weakest to strongest. If you think that two (or more) diagrams should be ranked as equal, drag one on top of the other(s) to show this equality. (Not to scale)

Weakest- one Jupiter mass and 2 Au Same- Four Jupiter Masses 2 AU & 1 Jupiter mass 1AU Strongest- 2 Jupiter masses 1 AU

Is it possible to determine the planet's mass from the star's velocity curve?

Yes, by measuring both the star's orbital period and its change in velocity over the orbit

In general, which type of planet would you expect to cause the largest Doppler shift in the spectrum of its star?

a massive planet that is close to its star

The following four graphs show the velocities, toward and away from Earth, of four identical stars (the same graphs shown in Part A). Assume that all four stars have extrasolar planets orbiting them at the same distance. Rank the graphs from left to right based on the amount of time that it takes the extrasolar planet orbiting each star to complete one orbit, from shortest to longest. If you think that two (or more) of the graphs should be ranked as equal, drag one on top of the other(s) to show this equality.

all the same

Suppose a planet is discovered by the Doppler technique and is then discovered to have transits. In that case, we can determine all the following about the planet except ______________.

its rotation period

The following four graphs show the velocities, toward and away from Earth, of four identical stars. Assume that all four stars have extrasolar planets orbiting them at the same distance, and that the velocities are inferred by measuring Doppler shifts in the spectra of the stars. Rank the graphs from left to right based on the amount of orbital Doppler shift observed in each star's spectrum, from smallest to largest.

lowest velocity wavelengths to highest velocity wavelengths.

The transit method allows us in principle to find planets around __________.

only a small fraction of stars that have planets

The following four graphs show the velocities, toward and away from Earth, of four identical stars (the same graphs shown in Parts A and B). Assume that all four identical stars have extrasolar planets orbiting them at the same distance. Rank the graphs from left to right based on the mass of the extrasolar planet that orbits the star, from smallest to largest.

same as answer number 7

Very few of the known extrasolar planets have sizes as small as Earth. The most likely reason for this fact is that ________.

small planets are more difficult to detect than larger planets