AST 111 Exoplanets I

Exoplanets Exoplanets: Planets orbiting stars other than the Sun

Exoplanets Remember: 99.9% of our Solar System is in the Sun Stars are so far that it is very difficult to image their planets Light from stars 1,000,000,000x stronger than from planets, and the starlight gets blurred Only 10 have been directly imaged

A Picture From the book: 1 to 10 billion scale: The Sun is a grapefruit Earth is the head of a pin 15 meters away Jupiter is a marble 80 meters away Distance to nearest star is distance across U.S. You’re in San Francisco looking at Washington D.C. The grapefruit is hard enough to find! Good luck with the pinhead!

Exoplanets But technology now moves at an astounding rate. As I was writing this, I received this!! Credit: ESO

Exoplanets YOU CAN OBSERVE EXOPLANETS. A 10” scope with a CCD camera is all that’s needed. ($4k) It’s even a homework problem in the book: Chapter 13, Problem 54

Exoplanets Before 1990’s, all we knew about planets was our own Solar System 1200 exoplanets (and more every day!) have been counted

Lots of Earths!

First exoplanet discovered (1995) 51 Pegasi First exoplanet discovered (1995) Surface temp of 1340 oF

How do we find them? Exoplanets are indirectly observed by: Gravitational wobble Astrometric technique Doppler technique Transits and eclipses Direct detection

Gravitational Wobble Objects in a solar system orbit the center of mass The star appears to “wobble” CM just outside Sun’s visible surface.

Gravitational Wobble Larger mass planets cause more wobble More planets add their own wobbles to the star Our Sun: Jupiter creates a wobble Saturn’s effect is perceptible Other planets’ effects hard to see

Gravitational Wobble (Astrometric) Astrometric method: Precisely measure stellar position 10 LY away: Large planet at 5AU from Sun-like star causes the star to move “the width of a human hair… seen at 3 miles” Best for massive planets orbiting close stars Takes many years of observing– ONE exoplanet found with this method

Gravitational Wobble (Doppler) 51 Pegasi discovered by alternating blue and red shifts Recall spectra: Blueshifts if moving toward us Redshifts if moving away

Gravitational Wobble (Doppler)

Gravitational Wobble (Doppler) Doppler technique accurate to 1 m/s Period of graph is the planet’s orbital period Kepler’s laws give distance from star

Gravitational Wobble (Doppler) Can determine orbital shape More elliptical orbit, more “skewed” graph

Gravitational Wobble (Doppler) The Doppler technique does not work from bird’s eye view! Recall that there must be relative motion toward or away from observer.

Gravitational Wobble (Doppler) Works best with more massive, closer stars More gravitational pull More wobble Less time wait for doppler shifts

Mass Estimate Conservation of Momentum: Mstarvstar = Mplanetvplanet Mplanet = Mstarvstar vplanet Doppler technique gives vstar. Know Mstar. _____________

Mass Estimate Kepler’s Laws give the radius of orbit Doppler effect gives period of planet’s orbit Also, vplanet = 2paplanet pplanet Mplanet = Mstarvstarpplanet 2paplanet ___________ ____________________

Mass Estimate 51 Pegasi: Mass of 51 Pegasi works out to 0.47 MJupiter 2.12 x 1030 kg (star’s mass) 57 m/s (star’s velocity) Radius of orbit: 7.82 x 109 m Planet’s orbital period: 3.65 x 105 seconds Mass of 51 Pegasi works out to 0.47 MJupiter

Transits and Eclipses Transit: Planet passes in front of star Eclipse: Planet passes behind the star Can’t actually see the dot moving across the star Measure brightness changes

Transits and Eclipses Requires correct alignment (edge-on)

Transits and Eclipses Amount of “dip” in brightness gives planet’s radius Have mass from Kepler’s Laws Can get density: Looks like terrestrial? Looks like Jovian?

Transits and Eclipses Can show composition of upper atmosphere During transit, starlight passes through the planet’s atmosphere

Planet Size From brightness measurements: Fraction of light blocked = area of planet’s disk = pr2planet area of star’s disk pr2star = r2planet r2star ________________________________ ________________ __________

Example: HD209458 rplanet = rstar x (fraction of light blocked)1/2 Star HD209458: Radius of 800,000 km Planet blocks 1.7% of light (0.017) rplanet = 800,000 km x (0.017)1/2 rplanet =100,000 km

Transits and Eclipses One can observe the system in infrared during eclipse Planets glow in infrared Decrease in IR part of spectrum shows how much IR the planet gives Can calculate planet’s temperature Can identify greenhouse gases

Direct Detection Works for very large planets Far from parent star

Direct Detection

Direct Detection

Exoplanets From the book: Although it is too soon to know for sure, it seems ever more likely that our Milky Way Galaxy is home to billions of planetary systems.