1445 Introductory Astronomy I

1. 1445 Introductory Astronomy I Chapter 5b Extrasolar Planets R. S. Rubins Fall, 2010

2. Extrasolar Planets 1 • As of July 2010, the count of extrasolar planets, orexoplanets,stood at more than 450, and is growing rapidly. • Because it is much easier to detect large planets, most observations up to this time have been confined to gas giants. • Orbital periods have ranged from 1 to 13,000 days, and planet sizes have ranged from a 2010 observation of a 1 ME planet to numerous observations of planets of roughly 10,000 ME (or about 4 Jupiter masses). • Many of the observed exoplanets are known as hot Jupiters, which orbit their stars at distances much closer than Mercury’s average distance from the Sun (0.4 AU).

3. Extrasolar Planets 2 • In October, 2008, the hottest planet yet observed, Wasp-12b, was found to have1.5 times the mass of Jupiter and almost twice its radius, and a surface temperature of about 2200 oC (4000 oF). • Since Wasp-12b orbits its star in about 1 Earth day it must be only about 2 million miles(0.02 AU) from its star, i.e. 1/20 the distance of Mercury from the Sun. • Reported in 2009, was the first planet orbiting a binary star system, in which the two stars are just 5 solar diameters apart. • In 2008, carbon dioxide (CO2), which is one of the biomarkers of a habitable planet (along with water, oxygen and HCN), was found in the atmosphere of one exoplanet.

4. Planets of Upsilon Andromeda and the Sun

5. Hot Jupiters • Until recently it was thought the planets remained in orbit at about the same distance from the Sun as they were originally formed. • Mathematical models indicate that in a dense disk, large planets can spiral inwards as they sweep up planetesimals. • Hot Jupiters are thought to be formed in the outer regions of their disk, before migrating to roughly (1/10)th of Mercury’s distance from the Sun. • The cloud-top temperatures of over 1300 K cause the outer layers to expand, making such planets larger and less dense than Jupiter.

6. Comparison of Jupiter & Hot Jupiters

7. The First Kepler Exoplanets • In 2010, NASA’s Kepler Space Telescope detected its first five exoplanets, named Kepler 4b through 8b. • They orbit their parent stars in 3 to 5 days, and have surface temperatures between 1200 and 1650 oC (2200 to 3000 oF), which is hotter than molten lava. • Kepler 7b has an average density about that of styrofoam.

8. The Problems of Space Travel • If a ship could be made to travel, for example, at 99.9% of the speed of light, the 50 ly round-trip to the region of the star Vega would take just two years because of length contraction, which follows from Einstein’s Special Theory of Relativity (1905). Two major problems • In the above example, people on Earth would have aged by 50 years in the 2 years taken by the space travelers, so (for example) their children would be older than they on their return. • A practical problem is that to accelerate a spaceship of the size of Star Trek’s Enterprise to just half the speed of light would require roughly 200 times the Earth’s annual energy expenditure.

9. Habitable Planets 2 The basic requirements for life to exist on a planet are the following: 1. liquid water;2. nutrients;3. a low-entropy energy source. The Earth has supported intelligent life in part because • the Sun is a longer-lived smaller star, which allows billions of years for evolution to do its work; • the climate is temperate, and there is a huge amount of surface water; • there is a sufficient quantity of heavier elements created by the explosions of earlier generations of stars; • there is a single moon large enough to stabilize the Earth’s motion and play an important role in creating tides. • the Sun is a hot source in a cold background sky, which provides low-entropy photons.

10. Finding Extrasolar Planets • Since exoplanetsare small, both the small amount of light they reflect from their companion star, and the weak infrared light emitted by their relatively cool surfaces, are overwhelmed by the radiation emitted by the star. • Thus, prior to the first direct imaging of exoplanets in 2008, only indirect methods had been used in the search for exoplanets. • The most common methods used are the following: 1.the transitmethod; 2. the radial-velocity(or Doppler) method; 3. the astrometricmethod; 4. microlensing; 5. direct imaging.

11. Transit Method • In the transit method, the passage of the unseen planet in front of its star causes a slight diminution of the light intensity observed from the star. • The shape of the light curve (right figure) allows the sizes of the orbit and the planet to be estimated. • As of October 2008, about 50 planets have been discovered by this method.

12. A star and its planet orbit about the center of mass. As thestar moves across the Sky its motion about the center of mass gives it a wavy motion. Motion about the Center of Mass

13. The Radial Velocity Method 1 • The motion of the star about the center of mass due to the planet’s orbit causes the light it emits to show a Doppler effect which alternates between a redshift and a blueshift. • The measurement of small Doppler shifts of this type indicate the presence of a planet.

14. The Radial Velocity Method 2 • (a) Data showing fluctuations of 100 m/s in the speed of the Sun-like star 51 Pegasi, which is 40 ly away. The data indicate the presence of a planet with a mass of half that of Jupiter, and a period of 4.2 days. • (b) Complicated data, showing the effect of three planets, with minimum masses of 0.7, 2.1 and 4.3 times that of Jupiter.

15. The Astrometric Method • The wavy motion (or wobble) in the star’s motion across the sky may sometimes be observed directly by telescopic observation.

16. Microlensing 1 • When a dark object of sufficient mass passes between a distant star and ourselves, the star is not darkened, as might be expected. • Instead, an important effect of Einstein’s Theory of General Relativity - the bending of light by a gravitational field – causes the dark object to act as a lens, which focuses the light, and makes the star appear brighter.

17. Microlensing 3 (1) No microlensing (2) Star only (3) Both star and planet

18. Direct Imaging 1 • An infrared photo taken by the VLT telescope in Chile, using adaptive optics, shows a possible exoplanet (in red) at a distance of 55 AU from the brighter object, which is a brown dwarf (a failed star). • Further measurements over time should show if the planet is orbiting the star.

19. Direct Imaging 2 • In November 2008, two sets of direct observations of exoplanets were reported. • One set of observations was made with the Hubble space telescope, and obtained images of the coolest and lowest-mass exoplanet then observed, orbit in about the star Fomalhaut, which is about 25 ly away, and one of the brightest stars in the southern hemisphere. • At a distance of about 100 AU from the star (about three times the distance of Pluto from the Sun), the planet Fomalhaut b is much too cold to support life. • In the other, IR images of three planets, ranging from 5 to 13 times the mass of Jupiter, were observed near the star HR 8799, which is about 130 ly away.

20. Fomalhaut b 1 ↓ • The exoplanet Fomalhaut b is the lowest-mass (and coldest) planet observed outside the solar system.

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

22. Comparison HR 8799 and Solar System • The orbits of the planets observed around HR8799 are similar in size to those of the gas giants in our system.

23. In a binary-star system, do planets orbit one or both stars? A NASA study indicated that a planet orbits both stars if they re separated by less than 3 AU, and just one, if more than 50 AU. In 2009, an exoplanet was found to orbit stars 5 AU apart. Planets and Binary Stars