PHYS Distilling the Planets - Observational Data Mercury, Venus, Earth and Mars all have an iron core, rocky silicate crust and a thin atmosphere, almost no hydrogen or helium. Jupiter and Saturn are much larger, made mostly of hydrogen and helium, have a thick gaseous atmosphere and small rock/iron core. Uranus and Neptune are about 15 times the mass of Earth, made of rock and volatile ice. Space Junk between Mars and Jupiter:Asteroids are lumps of rock or nickel/iron. Some are chondrites with millimetre-sized spherical silicate granules called chondrules which appear to have been flash melted and frozen. beyond Pluto: Comets are lumps of dirty ices, water, carbon dioxide, methane etc.

PHYS Distilling the Planets - a Model 1. Heat from the protostar probably destroys gas molecules, but in the disc they can be recreated more easily due to high density. 2. New dust grains condense out of the gas. 3. Tens of AU from the Sun the temperature is below 150 K (-123 o C), so volatiles can condense to ices, H 2 O, carbon dioxide, alcohols etc. (1 AU = Earth - Sun distance) 4. Within 5 AU the temperature reaches 1400 K and only silicates, iron and rocky materials can condense. 5. Violent electrical storms may have occurred within 5 AU to create chondrules. 6. Particles grow by attraction and collisions - this theory is not yet secure.

PHYS Distilling the Planets - a Model 7. In a few hundred thousand years the lumps are up to a few tens of kilometres across. 8. These bodies continue to collide, eventually forming the planets we see today. Some of the collisions must have been extremely violent - the Moon is thought to have been created in a collision between Earth and a Mars-sized object. 9. Jupiter and Saturn grew in the same way, but also collected the ices that had condensed and the clouds of hydrogen and helium gas which solar radiation had removed from the inner solar system. 10. Neptune and Uranus grew more slowly as the density of icy planetesimals was lower, and by the time they were heavy enough to attract gas, the solar wind and radiation had flushed it clear of the Solar System.

PHYS Planet Hunting - Dust Discs 1984: InfraRed Astronomical Satellite, IRAS saw extended infrared emission surrounding the star Beta Pictoris. IR emission most likely due to dust particles warmed up by light from the star. Ground-based telescopes confirmed that Beta Pictoris had an edge-on disc reaching 400 AU from the star. 1997: The Beta Pictoris disc is found to be warped - this may be due to the gravitational pull of one or more planets… … but could be the effect of a close encounter with another star or a dense cloud of young comets. Figs. Z22.14 & K : Three separate dust rings are mapped - could be held apart by planet(s).

PHYS Planet Hunting - Reflex Motion Planets too faint and too small to be seen directly make the star they orbit wobble. Look at star’s position - astrometry Look at star’s spectrum - Doppler shift Big planets have strongest effect on star, so are likely to be found first. Astrometry looks for star moving from side to side - easiest to detect for nearest stars with large planets far from the star. Doppler shifts are biggest for large planets close to the star. Biggest shift can be observed if the orbit is viewed edge-on, such that we measure its true “Radial Velocity”. Jupiter causes the Sun to move around a circle of radius about 1 / 100 th of Mercury’s orbit at a speed of 45 km per hour.

PHYS Planet Hunting - 51 Pegasi 10/1995: Mayor and Queloz at the Haute- Provence Observatory found a Doppler shift with a Sun-like star, 51 Pegasi. Measured velocity of up to 216 km per hour Orbital period of candidate planet 4.2 days Mass about half that of Jupiter Just 0.05 AU from star ( 1 / 20 th of Earth-Sun) Surface temperature probably about 1300 K Confirmed by Marcy and Butler Nothing like Mercury / the solar system. How did it get there? Massive planet formed further out and dragged in by gas and dust? If so, any terrestrial planets would have been kicked out into interstellar space - not good for life as we know it!

PHYS Planet Hunting - Radial Velocity ~ 300 planets discovered with this technique 1997: 16 Cygni B - a “classical Jupiter” planet found around a Sun-like star - but highly eccentric orbit, whereas solar system planetary orbits are very close to circular. 2001: 47 Ursae Majoris - planetary system. Has a 2.6 Jupiter mass planet in a near circular orbit at 2.1 AU from the star. If this were in our solar system it would lie between Jupiter and Mars and might prevent an Earth-type planet forming in the “habitable zone”. 04/2009: Gleise 581e - smallest exoplanet around a normal star. Minimum 1.9 Earth masses. Just 0.03 AU from parent red dwarf star in a 3.15 day orbit and therefore outside the “habitable zone”.

PHYS Planet Hunting - Transits ~ 70 planets discovered with this technique A planet passing directly in front of a star along our line of sight blocks its light and reduces the star’s apparent brightness. Relative change in = Area of planet brightness Area of star For an Earth-like planet brightness drops by 0.01% for a few hours in a year. Can measure orbital period and physical size of planet. Likelihood of transit depends on viewing geometry - 0.5% if Earth-like. Easiest planets to detect are very large and close to the star - “Hot Jupiters”. The Kepler mission launched on 6/3/09 will stare at a patch of sky containing 100,000 target stars for 3.5 years. See

PHYS Planet Hunting - atmospheres As transit begins starlight filters through the planet’s atmosphere and we can look for spectral absorption lines. 2004: oxygen and carbon detected around a 0.7 Jupiter mass planet orbiting its parent star at 0.04 AU - thought to be the core of an evaporating gas giant. Occultation: when a planet moves behind it’s star we see only star light. If we subtract this spectrum from the light measured when planet and star are side by side the tiny difference is emission from the planet. 2005: first detection of a planet’s thermal emission. Used Spitzer Space Telescope’s infrared camera so the star was only 400  brighter than the planet (would swamp it by  10,000 in visible light). Want to search spectra for “biomarkers” - methane, ozone, water…

PHYS Planet Hunting - Imaging Direct Imaging is difficult because (i) stars are typically a million to ten billion times brighter than the planet. Need to use a coronagraph to block out the star so light reflected by the planet can be seen. (ii) on an astronomical scale planets are very close to their parent stars so a high resolution is needed to separate them. 2008: First visible light image of an extrasolar planet, Fomalhaut b, recorded by Hubble Space telescope. Infrared images of a 3 planet system taken with 8m ground- based telescopes. In both cases the motion of the planets shows they are orbiting objects. Detection of extrasolar planets is a goal of the European Extremely Large Telescope - a proposed 42m dish costing €960 M.

PHYS Planet Hunting - Status As of 20th October 2009 more than 400 extrasolar planets have been identified. See 09/2002: radio telescope detects water molecules in the Upsilon Andromedae planetary system. 10/2002: a planet is detected in a binary star system - most stars are in binaries, so possible sites of planet formation greatly increased. 05/2007: Gleise 436b - this planet’s density is found to be consistent with a 50% rock + 50% water composition (but its orbit is eccentric). 02/2008: Spitzer measures warm dust around other stars indicative of colliding rocky bodies in orbits of 1 to 5 AU - suggests at least 20% of Sun-like stars in our galaxy (~ 5 billion stars) could have rocky planets.