Why is it hard to detect planets around other stars?

Transcription

1 Extrasolar planets

2 Why is it hard to detect planets around other stars? Planets are small and low in mass Planets are faint The angular separation between planets and their stars is tiny

3 Why is it hard to detect planets around other stars? Planets are small and low in mass Even Jupiter has only about 1% the surface area of the Sun, and 0.1% of the mass

4 Why is it hard to detect planets around other stars? Planets are faint Planets do not emit their own light (at least at visible wavelengths). They simply reflect the light from the star which means they can be 1 billion times fainter than the star. The reflected light from the planet is drowned out by the light from the star. They do emit light according to their temperature (the thermal spectrum), but since they have low temperatures all of their light is in the infrared.

5 Why is it hard to detect planets around other stars? The angular separation between planets and their stars is tiny If the nearest star, Proxima Centauri, is at a distance of 4.4 light-years and if it had a planet orbiting it with an orbital radius of 1 AU, then the angular separation between the planet and the star would be 0.7 arcseconds - This is smaller than the thickness of a credit card viewed from across a football field

6 So how can we ever know if other stars have planets about them? We primarily detect them indirectly. But in some cases we re can actually see them

7 So how can we ever know if other stars have planets about them? We primarily detect them indirectly. But in some cases we re can actually see them

8 So how can we ever know if other stars have planets about them? We primarily detect them indirectly. But in some cases we re can actually see them

9 So how can we ever know if other stars have planets about them? Three methods of indirect detection: The astrometric method the position of a star shifts periodically The Doppler method we see a periodic Doppler shift in a star s spectrum Transit method the planet passes in front of the star

10 So how can we ever know if other stars have planets about them? The astrometric method First realize that a planet doesn t really orbit around a star actually they both orbit about their mutual center of mass.

11 So how can we ever know if other stars have planets about them? The astrometric method First realize that a planet doesn t really orbit around a star actually they both orbit about their mutual center of mass. So in some cases, even when we can t see the planet, we can see a star wobble. Then we can infer the presence of a planet.

12 So how can we ever know if other stars have planets about them? The Doppler method Recall that the wavelength of light increases (and the frequency decreases) for an object moving away from you. And vice-versa for an object moving towards you.

13 IF_10_03_doppler_shift_binary_system.htm

14 So how can we ever know if other stars have planets about them? The Doppler method Recall that the wavelength of light increases (and the frequency decreases) for an object moving away from you. And vice-versa for an object moving towards you.

15 So how can we ever know if other stars have planets about them? Transit method If the plane of the orbit is viewed edge-on, then the planet will pass in front of the star, making it slightly dimmer.

16

17 What do each of these methods tell us? Both the astrometric and the Doppler method give us information about the planet s mass; a more massive planet will cause the star to wobble more. Planetary transits tell us about a planet s size; a larger planet will block out more of the light. If we can detect a planet using both techniques (by detecting a wobble as well as a transit) then we can figure out the density of a planet then we can make a guess as to what it s made of.

18 What are the drawbacks to these methods? But these methods all have drawbacks The amount of wobble in the astrometric method depends on mass of the planet; we can easily find massive planets, but it s very hard to find low-mass planets. Additionally, this method is just really hard. The Doppler method also depends on the planet s mass. Additionally, this method works best when the orbit is viewed edge-on. The transit method only works when the planet s orbit is viewed edge-on; this is only about 1% of the time.

19 What are the drawbacks to these methods? We ve found thousands of planets so far, but many of them can be classified as hot Jupiters: they are very massive (otherwise they would be difficult to detect) they are very close to their stars (otherwise they would have a long orbital period, and so it would be difficult to see the wobble/transit) There may be many Earth-like planets out there, we just haven t been able to learn much about them yet

20 Masses and sizes of extrasolar planets

21 Do the hot Jupiter s pose a challenge to our theory of planet formation? According to our theory, planets condense out of the protoplanetary disk Inside of the frost line, only rocks and metals can condense into planetesimals, and hydrogen compounds (including water) remain in a gaseous state. But beyond the frost line hydrogen compounds can also condense into ices.

22 Do the hot Jupiter s pose a challenge to our theory of planet formation? According to our theory: There simply isn t much rocky material and metals around, so the planetesimals inside the frost line never become very big -> so the inner terrestrial planets are relatively small and rocky However the outer planetesimals have a lot more hydrogen, helium, and hydrogen compounds that can condense. So they can become much more massive, and due to their gravitational force can start attracting even more material -> so the outer jovian planets are large and contain mostly hydrogen, helium, and hydrogen compounds

23 Do the hot Jupiter s pose a challenge to our theory of planet formation? So the hot Jupiters are difficult to explain! So far we think that our theory is correct, and that the massive planets form far from their stars. But something happens that cause the planets to migrate inward. Perhaps the gravitational interactions between the protoplanetary disk and the recently-formed planets caused them to migrate inward before the disk got cleared out