Lecture 13

6/29/07

Astronomy 1001

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?

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

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

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

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?

Other Methods

• Direct detection– Difficult to impossible

due to the glare of the star

• Gravitational Lensing

• Disturbances in dust disks

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

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

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

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

Life in the Universe

• 20th 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

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

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

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

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

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

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

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

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

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

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

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?

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

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