Young Solar-like Systems FIG.2. Panels(a),(b),and(c)show 2.9,1.3,and 0.87 mm ALMA continuum images of other panels, as well as an inset with an enlarged view of the inner 300 mas centered on the (f) show the image and spectral index maps resulting from the combination of the 1.3 and 0 α/α error < 4. The synthesized beams are shown in the lower left of each panel, also see Ta corresponds to 2 rms to 0.9 the image peak, using the values in Table 1. The colorscales rms and image peak corresponding to each respective wavelength in Table 1.
The evolution of a Solar-like system
Solar System Jovian Planets + Icy Moons KBOs Frost Line Terrestrial Planets Sun hot cold
Searching for Extrasolar Planets: Motivation To determine how common solar-like systems are To determine how typical our solar system is To ultimately find other Earth-like planets
Methods of Finding Extrasolar Planets 1) Direct imaging 2) Transits 3) Star s wobble 4) Radial velocity
1) Direct Imaging Difficult to do. Problem 1: Light contrast between planet (which shines via reflected star light) & its Star. Typically, Problem 2: small angular separation Remember that 1 AU at a distance of 1 parsec (approximate distance of nearest star) corresponds to 1
2008 (Kalas et al. 2008) 0.05 to 3.0 Jupiter mass planet detected around the A-type star Fomalhaut (25 light years away) The possible presence of the planet was inferred earlier by the sharp inner boundary of the dust lane
2008 3 planets around the A-type star HR 8799 (130 light years away) planet star distances ~ 24, 38, 68 AU planet masses ~ 5-13 Jupiter Masses (Marois et al. 2008)
2) Transits I.e., the dimming of a star by eclipsing planets The size of the eclipsing object and its orbital period can be derived from these observations. (For Jupiters, size is < 1.4 Jupiter diameters) Problem 1 - unless the planet is large in size relative its star (as seen from our vantage point), this effect will not be significant
2) Transits Problem 2 - the orbital inclination relative to us must be such that we see the planet transiting the disk of the star
Transits - Solutions Look at lots of stars at once Look for a relatively long time Focus on low mass stars. The smaller the star, the stronger the signal from a transit event
Kepler Mission A 0.95 m diameter telescope which continuous observed a strip of size 10ox10o. Monitors the brightness of 100,000 stars brighter than 14th magnitude in the constellations of Cygnus and Lyrae
Example: Kepler-16: A transiting circumbinary planet Planet b Star A Star B The planet has a mass = 0.33 Jupiter masses and a radius 0.75 that of Jupiter The planet s density is 0.964 g cm -3
Example: Kepler-16: A transiting circumbinary planet
Both the star & planet move around a center of mass. The location of the center of mass depends on the mass of the star and the planet. 3) Wobble
What does wobbling look like to us?
4) Radial Velocities Accuracy needed: ~10s of m s -1 or ~ 20 mph (Video)
Radial Velocities
The Keck 10m Telescope is presently being used to find extrasolar planets, but the initial discoveries were made with small telescopes Butler
First, properties of some of the solar system planets Planet Distance from Sun (AU) Orbital Period (Earth years) Eccentricity Mass (Earth = 1) Mass (Jupiter = 1) Earth 1.0 1.0 0.017 1.0 3x10-3 Mars 1.5 1.9 0.093 0.11 3x10-4 Jupiter 5.2 12 0.048 318 1 Saturn 9.5 29 0.054 95 0.3 Note: Eccentricity of a circular orbit = 0.0
Our inner solar system
& some extrasolar planet systems
Earth Jupiter Exoplanets.org
Earth Jupiter Exoplanets.org
Minimum Mass Distribution orbital plane host star planet Exoplanets.org
Earth Jupiter Exoplanets.org
Mercury Earth Jupiter Saturn Exoplanets.org
Exoplanets.org
There has been a reported Extrasolar Planet with Measured Atmosphere Planet about 220 times the mass of Earth In orbit around a 7 th magnitude star 150 light years away 20 times closer to star than Earth-Sun distance
(Video)
Planet Eclipses its Star Atmosphere can be detected via absorption towards star Light of specific wavelength absorbed by atmospheric gas
Sodium Absorption Line Detected Atmospheric absorption feature will shift as planet orbits star Why? The Doppler effect.
Extrasolar Planets The most successful methods of finding exoplanets to date: the radial velocity and transit techniques. The majority of extrasolar planets found to date are very massive & very close to the host star This is quite different from our solar system Note the observational bias the most successful techniques favor the identification of massive planets close to their host star But the techniques wouldn t be successful if massive planets orbiting near their host star weren t common!
How do we know they are gas giants? Sizes can be measured from transit events. E.g., the size of one was measured from an eclipse event to have a diameter 1.35 times that of Jupiter. (See also Kepler l6 example) Density ~ 0.6 Jupiter density If these are typical, then the extrasolar planets are gas giants
How do you get such massive gas giant planets to form near their stars? I.e., our gas giants formed beyond the frost line. The abundance of ices (I.e., of Hydrogen & Oxygen) + cold temperatures + cosmic proportions of Hydrogen & Helium = gas giants Most likely explanation for these planetary systems = planet migration The gravity of planets on the circumstellar disk can create waves that propagate through the disk. These waves can steal angular momentum from planets as they pass, causing them to spiral inward. This phenomenon must occur before the circumstellar material is cleared Infall may be halted when the planet is close enough to the star to be tidally locked (< 0.1 AU). What about those at intermediate star-planet distances??
What about terrestrial-like planets in these systems? It s unlikely that rocky planets could form within the frost line with gas giant planets so close to their star, especially if migration has occurred Disk material would be stirred by the gravity of the gas giant planet, thus disrupting accretion into rocky planets However, some of these gas giant planets could have moons that are rocky
How many Earths are there in the Milky Way that could be Inhabited? Best guess ~ between 1 & several hundred million Condition 1: Star must be a single star Condition 2: Star must be rich in heavy elements Condition 3: Star must reside in a place where there are few supernovae Condition 4: Star must spend ~ 4 billion years steadily burning fuel at constant luminosity (if Earth is a good example!!!!) 2 out of 34 stars close to the Sun meet these conditions Extrapolating, 15 billion stars qualify Probably 10% of Sun-like stars have giant planets (based on extrasolar planet searches) Unknown - the number that have gas giants only beyond the frost line.
Other factor - the Habitable Zone Habitable Zone: the region around a star where a planet (or moon) can maintain water in liquid form. The distance of the HZ from the host star depends on the temperature of the star too warm too cold just right
Fermi Paradox: So where is everybody? There are billions of stars in the Galaxy similar to our Sun. Many are billions of years older. Some of these stars have planets like the Earth, and thus some fraction must have had intelligent life develop on them Some of these older civilizations could have completely populated the Milky Way within a few million years (with ships traveling a few percent the speed of light) Thus, the Earth should have been visited by extraterrestrial life. Where are they??
Fermi Paradox: Possible Solutions 1. We are alone 2. Civilizations are common, but no one has colonized the galaxy due to The expense of such colonization A lack of desire to explore The inclination of technologically advanced civilizations to destroy themselves 3. These extraterrestrials have not yet revealed themselves to us.
Drake Equation Frank Drake (1961) - specific factors that play a role in the development of technologically advanced civilizations. I.e., how many are there? SETI - the Search for ExtraTerrestrial Intelligence
Drake Equation N = R f p n e f l f i f c L N = The number of civilizations in The Milky Way Galaxy whose electromagnetic emissions are detectable. R * =The rate of formation of stars suitable for the development of intelligent life. fp = The fraction of those stars with planetary systems. ne = The number of planets, per solar system, with an environment suitable for life. fl = The fraction of suitable planets on which life actually appears. fi = The fraction of life bearing planets on which intelligent life emerges. fc = The fraction of civilizations that develop a technology that releases detectable signs of their existence into space. L = The length of time such civilizations release detectable signals into space.
Breakthrough Listen 100m Green Bank Telescope 64m Parkes Telescope Automated Planet Finder Lick Observatory Program designed to examine 1 million stars in the galactic center and the galactic plane, as well as the nearest 100 galaxies, for radio or laser signals produced by extraterrestrials. The instruments used are 50 times more sensitive than those used in previous SETI searches, and the area searches is 10 times larger http://breakthroughinitiatives.org/initiative/1