1. Lecture 13 6/29/07 Astronomy 1001

2. Difficulties It is extremely difficult to detect extrasolar planets Stars are a billion times brighter than the reflected like would be How exactly should you define a planet anyway?

3. Early Discoveries A planet around 51 Pegasi was discovered in 1995 –Over 200 planets have now been discovered We detect planets using indirect methods now –Will hopefully master direct detection in the next decades

4. Gravitational Tugs Jupiter (and all of the other planets) exert a gravitational tug on the planets We can use the Astrometric Technique to simple watch the position of the star changing We can use the Doppler Technique to observe the effects of the star moving –Can figure out lots of properties of the orbit using physical laws

5. Transits A third method is to look for transits –The planet will block out some of the star’s light, so we can tell its size Can also do some spectroscopy to figure out the composition of the atmosphere

6. Group Work Lets say that you wanted to detect a planet with an Earth like radius (6500 km) around HD209458. What percentage of the star’s light would be blocked, and do you think that its plausible that a telescope could detect this transit?

7. Other Methods Direct detection –Difficult to impossible due to the glare of the star Gravitational Lensing Disturbances in dust disks

8. Properties of Extrasolar Planets The planets we can detect are typically large and close to their host stars –Unlike any planets in our own solar system –Not many Earth-like planets discovered Gliese 581 appears to have a terrestrial planet in the habital zone

9. Solar System Formation The current model of planet formation makes several predictions –Jovian planets should only form far away from their stars –Planets should form more often around stars with lots of metals and rock Jovian planets have probably migrated inwards

10. The Future We want to detect Earth- like planets Kepler and COROT will both star at the same patch of sky and look for brightness changes –Kepler will look at ~100,000 stars for 4 years –COROT already launched

11. Other Missions GAIA and SIM –Extremely precise astrometric method devices TPF and Darwin –Designed for direct detection of planets –Will block out light from the star –Scheduled to launch in 2015-2020

12. Life in the Universe 20 th Century observations of many planets caused the topic of life in the Universe to wane But there is renewed interest in thinking about life in the Universe –Life arose very early in Earth’s history –Lab experiments indicate that you can form the building blocks of life very easily –We have found organisms that can survive in conditions similar to those on other worlds –We are beginning to find many planets outside of the solar system

13. Early Life Heavy bombardment ended about 3.9 billion years ago We have strong evidence of life at 3.5 billion years ago –Advanced stromolites have been radiometrically dated to that time

14. The Rise of Life Species have evolved over time DNA was a key discovery –Contains genetic information We have found organisms that can survive in extreme heat We have produced RNA in labs, as well as the necessarily amino acids for DNA

15. A History of Life Panspermia is the theory that life evolved on other planets and migrated here Life started off aquatic and single celled and remained that way for 2 billion years Eventually the ozone layer was formed and life moved to land Large life evolved rapidly beginning 540 million years ago

16. What is Necessary for Life? Life exists in such a wide range of situations here on Earth that the necessities of life are fairly broad –Need a source of nutrients –An energy source –Liquid water

17. Life on Mars Despite repeated experiments, no evidence for life on Mars has ever been found –Future missions will explore more hospitable locations Evidence from Martian meteorites is inconclusive –ALH84001 created some hype a few years back

18. Life Elsewhere in the Solar System Europa represents the most likely other location for life Titan has been more or less ruled out due to its recent visit from Huygens Other moons seem less likely candidates

19. Life Around Other Stars Life could very well exist around other stars if: –The star is old enough –Planets orbit probably has to be stable –Planet must be in the habitable zone This means that there are billions of stars that could have planets or moons with life

20. Detecting Life The best we can do with images is to possibly spot oceans A better method of detecting life indirectly is to look at the composition of atmospheres –Recall that the oxygen in Earth’s atmosphere is due to life

21. How Rare are Earth-like Planets? Location in galaxy could potentially be important Jovian planets would have to be in the right location Climate stability is important –Plate Tectonics –Large moon

22. Extraterrestrial Intelligence It might be possible to detect broadcasts from other civilizations –SETI The Drake Equation estimates that there should be thousands of intelligent civilizations out there

23. Group Work Any reasonable values for the Drake Equation will estimate that there should be thousands of intelligent alien civilizations out there. But, none of them have ever contacted us. Given the probable validity of the Drake Equation, what might be the reason why we have never been contacted?

24. How SETI Works SETI astronomers use Radio telescopes to look for alien signals –Also use visible light telescopes to look for things like laser pulses Have sent out high energy transmissions to clusters of stars

25. Interstellar Travel Current probes will take essentially forever (compared to human time scales) to reach distant stars It would take 2000x the whole world’s energy output to propel a spaceship to half of the speed of light Any currently feasible method of propulsion doesn’t improve on the picture much