Lecture 12: Extrasolar planets Astronomy 111 Monday October 9, 2017
Reminders Star party Thursday night! Homework #6 due Monday
The search for extrasolar planets The nature of life on earth and the quest for life elsewhere are two sides of the same question: the search for who we are. Carl Sagan
Testing the nebular theory The physical processes that formed our solar system should be commonplace We can observe young stars with disks But do those disks form planets like those in our solar system? Or do we live in a unique place in the Universe? ASTR111 Lecture 12
Why search for extrasolar planets? A good scientific theory has to be tested, but we only have one Solar System and we cannot go back in time. We needed to find other extrasolar planets to prove the nebular theory of Solar System formation So we did
Extrasolar planets As of 10/1/17, astronomers know of 3671 confirmed extrasolar planets in 2751 systems--and counting! http://exoplanets.org
What is an extrasolar planet? A planet outside of our solar system orbiting a star other than our Sun First one discovered by Mayor in 1995 51 Pegasi b: 0.5 Jupiter mass planet orbiting a 1 Solar mass star with a 4 day period
Detecting extrasolar planets Direct Methods: Pictures or spectra of the planets Indirect Methods: Precision measurements of stellar properties (position, brightness, spectra) that reveal the effects of orbiting planets Doppler shift Microlensing method Planetary transits
Direct detection Difficult since the star is much brighter than the planet, but sometimes possible at infrared wavelengths. Brown dwarf star with a Jovian planet
Direct imaging Most detected planets are 1 to 10 times more massive than Jupiter; many are Hot Jupiters. Astronomers have directly imaged only a few exoplanets to date. Large future telescopes will be able to image more exoplanets. ASTR111 Lecture 12
Doppler technique Measures motion of star along our line-of-sight seen as shifts in the star s spectrum due to gravitational force of planet on the star.
Doppler technique We can see this effect in our own Solar System: Jupiter causes a Doppler shift of the Sun!
Hundreds of extrasolar planets have been found to date via Doppler monitoring
Planetary transits A transit occurs when a planet passes in front of its star, slightly dimming the light from the star.
Planetary transits Artist s rendering of HD209458 ASTR111 Lecture 12
KELT-9b The hottest planet ever discovered: KELT-9b s atmosphere is T=4600 K, as hot as some stars!
Kepler NASA satellite launched in 2009 Repeatedly imaged a small area of sky to search for transiting planets Discovered over 2,000 confirmed planets and another 5,000 unconfirmed planetary candidates! Discovery of exoplanets vs. time
Gravitational microlensing Planets can be discovered via gravitational distortion of the light from background stars Uses mass of planets to bend the light from a very distant star towards us Technique gives mass and orbital radius of the planet Hard to execute but large scientific return Easiest way to detect planets at large orbital radii
20 Gravitational microlensing
Is our Solar System rare? Planets are suspected to exist around 50-100% of stars! But very few of those extrasolar systems look like our Solar System.
Many extrasolar systems are unlike our Solar System Orbits are highly eccentric & close to their stars (this is a bias due to using the Doppler technique)
Kepler Orrery https://www.youtube.com/watch?v=gnzvvym6kkm ASTR111 Lecture 12
Characterizing Exoplanets HD 189733b doi:10.1038/nature06823 Wasp-19b http://dx.doi.org/10.1088/0004-637x/771/2/108
Characterizing Exoplanets HD 189733b ASTR111 Lecture 12 http://dx.doi.org/10.1088/2041-8205/747/2/l20
Masses of extrasolar planets Very few!
Hot Jupiters Orbit very close to host star Easy to find with transit and RV methods Not likely to harbor life
In-class assignment A hot Jupiter unofficially named Osiris has been found around a solar-mass star, HD209458 (its official name is much more boring: HD209458b). It orbits the star in only 3.5 days, decreasing the brightness of the star by about 2%. If the diameter of HD209458 is 1.4 x 10 6 km, what is the diameter of Osiris?
Nebular theory needs to be revised Traditional nebular theory of planetary system formation does not account for the discovery of Hot Jupiters Need to revise theory to explain recent observations
Migration hypothesis New theory explaining the existence of Hot Jupiters Still needs some work!
Earth analogs The holy grail of extrasolar planet studies Probably have characteristics similar to Earth More likely to harbor life because they live in the habitable zone, where liquid water can exist Rare!
Life on extrasolar planets Astrobiology is a relatively new field with the goal of studying life outside of the Earth No life found in Solar System or on extrasolar planets (yet) Some Moons in Solar System (e.g., Titan and Enceladus) may have potential to harbor life Requirements: water, habitable zone
Questions astronomers are still resolving Are there extrasolar planets similar to the Earth? How many? Why are most extrasolar planets Hot Jupiters? Are there extrasolar planetary systems similar to our Solar System? Is our Solar System common or rare? Is there life on planets other than Earth?
Some questions: What are four techniques that astronomers use to discover extrasolar planets? Which technique has resulted in the largest number of discoveries? Why? Why is it difficult to find Earth analogs? Would it be easier for one of these techniques than the others?