Detecting Extrasolar Planets Radial Velocity Measurements

First Proposed in 1952

Radial Velocity Variations https://upload.wikimedia.org/wikipedia/commons/5/59/Orbit3.gif

Radial Velocity Variations

Radial Velocity Variations Doppler shift allows detection of slight motion of star caused by orbiting planet

Radial Velocity Variations A plot of the radial velocity shifts forms a wave. Wavelength tells you the period and size of the planet’s orbit. Amplitude tells you the mass of the planet. Blueshift Redshift 4 days Doppler shift in spectrum of star 51 Pegasi - shows presence of large planet with orbital period of about 4 days.

www.physics.sfsu.edu/~gmarcy/planetsearch/7504/7504.html www.exoplanets.org

A Very Eccentric Planet

Question What exactly do astronomers look for when using the radial velocity method to detect extrasolar planets? A variation in the brightness of a star. A wobble in the motion of a star as it moves through space. A periodic shift in the absorption spectrum of a star as it moves towards then away from an observer while ‘wobbling’ about a center of mass displaced from its own center. The sudden appearance of a double image of a star as an unseen planet moves in front of it. Whether the planet is moving toward or away from us.

Detecting Extrasolar Planets Radial Velocity Measurements Microlensing

No Microlensing Figure 8-19 Microlensing Reveals an Extrasolar Planet (a) A star with a planet drifts across the line of sight between a more distant star and a telescope on Earth.

Microlensing By A Star Figure 8-19 Microlensing Reveals an Extrasolar Planet (b) The gravity of the closer star bends the light rays from the distant star, focusing the distant star’s light and making it appear brighter. More light is redirected towards telescope because star acts as a ‘gravitational lens.

Microlensing By Star AND Planet Figure 8-19 Microlensing Reveals an Extrasolar Planet (c) The gravity of the planet causes a second increase in the distant star’s brightness. Even MORE light is redirected towards telescope because star AND planet act as a ‘gravitational lens.

OGLE 2005-BLG-390L Gravitational Lensing by Extrasolar Planet Mass = 5.5 ME at R = 2.6 AU

Detecting Extrasolar Planets Radial Velocity Measurements Microlensing Direct Imaging

Imaging in the near-infrared Figure 8-20 Imaging an Extrasolar Planet This infrared image from the European Southern Observatory shows the star 2M1207 and a planet with about 1.5 times the diameter of Jupiter. First observed in 2004, this extrasolar planet was the first to be visible in a telescopic image. 2M1207 and its planet lie about 170 light-years from the Sun in the constellation Hydra (the Water Snake). (ESO/VLT/NACO) ESO image of planet of mass 1.5 MJ

GQ Lup and AB Pic M = 13.5 MJ at R = 275 AU M = 21.5 MJ at R = 103 AU

HR 8799 The three large planets of HR 8799 can be seen in the infrared photo. Infrared Visible light

Comparison HR 8799 and Solar System The orbits of HR8799’s planets are similar in size to those of the gas giants in our system.

Question Why is it better to search for extrasolar planets directly using infrared (IR) rather than visible radiation? It’s easier to build large IR telescopes than visible telescopes Stars don’t emit any IR radiation because they are too hot Civilizations on planets emit more waste heat (IR) than visible light Planets don’t reflect any visible radiation, only IR radiation. Planets emit more IR radiation than visible radiation relative to their parent star.

Detecting Extrasolar Planets Radial Velocity Measurements Microlensing Direct Imaging Imaging of Planetary Transits and Eclipses

“Seeing” Extrasolar Planets Figure 8-18 A Transiting Extrasolar Planet If the orbit of an extrasolar planet is nearly edge-on to our line of sight, like the planet that orbits the star HD 209458, we can learn about the planet’s (a) diameter, (b) atmospheric composition, and (c) surface temperature. (S. Seager and C. Reed, Sky and Telescope; H. Knutson, D. Charbonneau, R. W. Noyes (Harvard-Smithsonian CfA), T. M. Brown (HAO/NCAR), and R. L. Gilliland (STScI); A. Feild (STScI); NASA/ JPL-Caltech/D. Charbonneau, Harvard-Smithsonian CfA)

Kepler mission (NASA) http://kepler.nasa.gov/

Kepler Mission This space-based telescope will point at a patch of the Milky Way and monitor the brightness of ~100,000 stars, looking for transits of Earth- sized (and other) planets 0.95-m aperture  capable of detecting Earths Launch: March, 2009

Number of Earths to be detected Monitor 100,000 stars Assume orbit at 1 AU around a G star Probability of transit: DSun/1 AU = 7×105 km/1.5×108 km = 5×10-3 (i.e., 0.5%) Expected number of Earths: N = 5×10-3(105)  Earth = 500  Earth where Earth is the expected frequency of Earth-like planets

A Planetary System http://exoplanets.org/esp/upsandb/upsandb.shtml

Upsilon Andromedae http://antwrp.gsfc.nasa.gov/apod/ap990422.html http://exoplanets.org/esp/upsandb/upsandb.shtml http://www.exoplaneten.de/upsand/english.html

A Sample of Observed Planets Our Solar System First known extrasolar planet, 51 Pegasi First known extrasolar system, Upsilon Andromedae

Detected Extrasolar Planets Total – 3706 confirmed in 2796 systems – more than 1 planet in 612 systems Radial Velocity Measurements – 669 … most quite large (Jupiter-like) Microlensing – 54 Direct Imaging – 44 very large planets Imaging of Transits – 2392… Kepler candidates – 2245 ‘Earth-sized’ – 23 < 3 ME

Most Are ‘Hot’ Jupiters Hot Jupiters are the easiest extrasolar planets to detect via the radial velocity method, because the oscillations they induce in their parent stars' motion are relatively large and rapid, compared to other known types of planets. Hot Jupiters are thought to form at a distance from the star beyond the frost line, where the planet can form from rock, ice and gases. The planets then migrate inwards to the sun where they eventually form a stable orbit. In simulations planets up to two Earth masses were able to form in the habitable zone after the hot Jupiter passed through and its orbit stabilized at 0.1 AU. Due to the mixing of inner solar system material with outer solar system material from beyond the frost line, simulations indicated that the terrestrial planets that formed after a hot Jupiter's passage would be particularly water-rich. It is estimated that 3% to 4.5% of sun-like stars possess a giant planet with an orbital period of 100 days or less.

The first extrasolar planets detected used the radial velocity method and most are ‘Hot Jupiters’. Does this mean that our solar system is ‘an anomaly’? Question No… the radial velocity method is ‘biased’ since large planets very close to a parent star cause the largest spectrum shift and therefore are most likely to be detected. Yes … it’s now obvious that ‘we’ are very rare! No … we haven’t yet examined enough stars to see if they have ‘normal solar systems’. No … we can’t be sure that the radial velocity method really tells us that it’s an extrasolar planet that has been detected. Yes … the data doesn’t lie so astronomers need to come up with a better hypothesis of how solar systems form.

Question How can we explain the presence of extrasolar planetary systems with Jovian-sized planets at distances where we normally find terrestrial planets? They are brown dwarfs that were captured by their parent stars. They are massive terrestrial planets that formed close to their parent stars. The Jovian planets formed farther out and then migrated inward. We can’t … the measurements of their positions is wrong. They are Jovian planets that were spun out from their rapidly spinning parent stars.

Where We Have Looked So Far

Conclusions A total of 3706 ‘exoplanets’ (in 2796 planetary systems and 612 multiple planetary systems) have been identified as of March 15, 2018. Estimates of the frequency of systems strongly suggest that more than 50% of Sun-like stars harbor at least one planet. In a 2012 study, each star of the 100 billion or so in our Milky Way galaxy is estimated to host "on average ... at least 1.6 planets." Accordingly, at least 160 billion star-bound planets may exist in the Milky Way Galaxy alone!

Conclusions Most known exoplanets are giant planets believed to resemble Jupiter or Neptune. That reflects a sampling bias, since massive planets are easier to detect. Some relatively lightweight exoplanets, only a few times more massive than Earth (‘Super-Earths’), have been detected as well. Statistical studies indicate that they actually outnumber giant planets. Recent discoveries by Kepler include several Earth-sized and smaller planets and a handful that appear to exhibit other Earth-like properties.

Conclusions Discovery of Gliese 581 g, thought to be a rocky planet orbiting in the middle of its star's habitable zone, was claimed in 2010 and it could be the most ‘Earth-like’ exoplanet discovered to date. But the existence of Gliese 581 g has been questioned or even discarded by many astronomers; it is listed as unconfirmed at The Extrasolar Planets Encyclopaedia. Subsequently, though, the super-earth, Kepler-22b, was confirmed to be in the habitable zone of its parent star, Kepler-22, the first planet of its size confirmed to be in this zone.

The planet orbits a (K-type) star, Kepler-62, with a total of five planets. The star has a mass of 0.69 M☉ and a radius of 0.64 R☉. It has a temperature of 4925 K and is ~7 Gyr old. Given the planet's age, irradiance (0.41 ± 0.05 SE) and radius (1.41 ± 0.07 RE), a rocky (silicate-iron) composition with the addition of a substantial amount of water is considered plausible. Modeling studies indicate it is likely that a great majority of planets in its size range are completely covered by ocean.

Conclusions So how many planets might support simple or even intelligent life? Dr. Alan Boss of the Carnegie Institution of Science estimates there might be a "hundred billion" terrestrial planets in our Milky Way Galaxy, many with simple life forms. He further believes there could be thousands of civilizations in our galaxy. Most recently, Duncan Forgan of Edinburgh University has tried to estimate the number of intelligent civilizations in our galaxy. His research suggests there could be thousands of them.

Conclusions The data collected so far indicates that 0.3% of all the known exoplanets have the potential to be habitable and the percentage grows to 4.1% if possible habitable moons are included. This is likely an overestimate, because of the over 100 satellites in our Solar System, only Jupiter’s moon Europa, and, to a lesser extent, Saturn’s moon, Enceladus, are generally considered to be habitats for life, and even in this case, this life would likely resemble the relatively simple life found in Earth's hydrothermal vents, a far cry from intelligence!

Habitable Exoplanets Catalog Go to: http://phl.upr.edu/projects/habitable-exoplanets-catalog