Life in the Universe

• Midterm next Thursday

• We’ll have a review session on Tuesday; come to class with questions

• Homework due on Tuesday

Extrasolar planets

Most stars have planets around them. Sometimes we can actually see the planets (“direct detection”)

Extrasolar planets

But usually we infer their presence by their effect on the star, rather than actually seeing the planets (“indirect detection”)

• If the planet passes directly in front of the star then the star will get slightly dimmer — this is the transit method

Extrasolar planets

But usually we infer their presence by their effect on the star, rather than actually seeing the planets (“indirect detection”)

• If the planet passes directly in front of the star then the star will get slightly dimmer — this is the transit method

• By using either the astrometric method or the Doppler method we can watch the star move due to the gravitational pull of the planet

• IF_10_03_doppler_shift_binary_system.htm

Extrasolar planets

In many cases we can see evidence for multiple planets

Extrasolar planets

In many cases we can see evidence for multiple planets

Extrasolar planets

In many cases we can see evidence for multiple planets

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Q: Do you think there’s life elsewhere in the Universe?A. Yes, obviouslyB. No wayC. Who knows?

How common is life in planets?

Evidence suggests that life on Earth occurred very early on

The oldest fossils show that bacteria-like organisms were present over 3.5 billion years ago, with other evidence suggesting that life may have existed 3.85 billions years ago

The period of heavy bombardment — when the Earth was constantly being hit by large asteroids and comets ended 3.9 billion years ago. Life probably could not have lasted during this period, which means that life basically started as soon as it possibly could!

- This suggests that life does not arise due to some very rare and random process. Perhaps basic forms of life arise easily, if the right conditions are present.

- But using this same reasoning, sophisticated civilizations may be much more rare

How common is life in planets?

The period of heavy bombardment — when the Earth was constantly being hit by large asteroids and comets ended 3.9 billion years ago. Life probably could not have lasted during this period, which means that life basically started as soon as it possibly could!

- This suggests that life does not arise due to some very rare and random process. Perhaps basic forms of life arise easily, if the right conditions are present.

- But using this same reasoning, sophisticated civilizations may be much more rare

How common is life in planets?

There are many different kinds of life

All of the plants and animals are just a very small part of the tree of life here on Earth. We should realize that other forms of life are probably much more common throughout the Universe

Life can arise in seemingly inhospitable conditions

Extremophiles are organisms that live in extreme environments, for instance with very high temperature or pressure. Other organisms can live in very salty, dry, or acidic environments.

Life can arise in seemingly inhospitable conditions

The taligrade (a “water bear”) is about 1mm in length. It can survive in extreme conditions, including (for a while) in a near-vacuum.

Basic necessities for life1. A source of nutrients (organic compounds) to build

cells out of.- This part is easy, since organic molecules are even

found on asteroids and comets

2. A source of energy: either starlight, or heat from the planet itself.

- This part is also easy, since there are lots of stars around, and large planets should retain plenty of internal heat

3. Liquid water

Liquid water

• There is plenty of evidence that Mars had liquid water on it’s surface in the past, and it may well still have subsurface oceans. If so, there may be life!

• Some of moons of Jupiter may also have subsurface oceans, with the temperature maintained by tidal forces from Jupiter.- There is a very real possibility for life in the solar system,

although it would probably be small and primitive

Liquid waterThe search for civilized life on other planets begins with the search for planets in the habitable zone — they must be far enough from their star for water on the planet’s surface not to boil off, and be close enough for water not to freeze.

Liquid water

• Very few of the planets found so far seem to be in the habitable zone. Many of the planets are too close to their stars, since those are the easiest to find; but as our measurements improve we’ll be able to find more.

Liquid water

Very recently it was discovered that the nearest star, Proxima Centauri, has a planet that appears to be in the habitable zone

Liquid water

Very recently it was discovered that the nearest star, Proxima Centauri, has a planet that appears to be in the habitable zone

• The planet is very close to it’s star; it has an 11-day orbit, and the star is about 15% as bright as the Sun

• Because it is so close, it might be tidally-locked to the star

• Depending on what kind of atmosphere it has, the temperature may or may not be appropriate for liquid water

Which planets are habitable?

Just being in the habitable zone may not be enough for life on it’s surface. A planet probably also has to have:

• an appropriate atmosphere, which requires geological activity

• magnetic fields may also be necessary to keep the atmosphere from being blown away by a stellar wind, and to protect life from harmful radiation

Which planets are habitable?

One of the important characteristics of the Earth is that the climate has been very stable. A major cause of this is the greenhouse effect.

One of the important characteristics of the Earth is that the climate has been very stable. A major cause of this is the greenhouse effect.

• Visible light heats a planet’s surface, which then emits infrared light. This infrared light is blocked by the atmosphere; in a sense the atmosphere acts as a blanket

• This effect takes place on any planet with an atmosphere, but it has maintained a perfect balance on Earth to keep the climate stable (compare the Earth to Mars or Venus!)

Which planets are habitable?

Which planets are habitable?

• The basic conclusion is that small and primitive forms of life might be very common, but large/intelligent/civilized must be much less common. How much less common? Who knows!

• But with ~100 billion stars in the Milky Way, and with ~100 billion galaxies in the observable Universe, it seems difficult to believe that we’re unique.

Finding intelligent lifeSETI — Search for Extraterrestrial Intelligence — uses radio telescopes to search for signals from space.

But any signals emitted from other planets would have to be very powerful to be detected using our current technology. But the technology is improving…

Q: If we’re going to look for signals from planets around other stars, what kind of stars should we focus our search on?

A. Main sequence starsB. GiantsC. Supergiants

Q: If we’re going to look for signals from planets around other stars, what kind of stars should we focus our search on?

A. Main sequence starsB. GiantsC. Supergiants

Q: If we’re going to look for signals from planets around other stars, what kind of stars should we focus our search on?

A. O and B type starsB. A and F type starsC. G, K, and M type stars

Q: If we’re going to look for signals from planets around other stars, what kind of stars should we focus our search on?

A. O and B type starsB. A and F type starsC. G, K, and M type stars

What about interplanetary travel?

It’s really difficult, since even the nearest stars are lightyears away!

• Warp drive, other ways of traveling faster than light, probably aren’t possible…

• Reaching speeds comparable to the speed of light is essentially impossible using normal fuel: accelerating a ship the size of Star Trek’s Enterprise to half the speed of light takes 2000 times the total annual energy use today.

So if we’re ever going to reach nearby stars in a reasonable amount of time, we need to come up with some completely new technology. For instance: Project Orion

What about interplanetary travel?

So if we’re ever going to reach nearby stars in a reasonable amount of time, we need to come up with some completely new technology. For instance: Project Orion

What about interplanetary travel?

Freeman Dyson

Are there other ways we can find life?

The most promising way to find evidence of life in the near future is by taking transit spectroscopy of stars

Are there other ways we can find life?Some small fraction of the star’s light will pass through the planet’s atmosphere

By analyzing the absorption lines in the star’s spectrum as the planet passes through it, we can gain information about the chemical composition in the planet’s atmosphere

If we see certain kinds of compounds in the atmosphere (for instance, a lot of oxygen) then that is a strong hint that life exists!

Are there other ways we can find life?Maybe the most promising way to find life in the near future is by taking transit spectroscopy of stars

Our current telescopes can’t make these measurements, but we are currently building the Giant Magellan Telescope, to begin operation in the mid-2020s

Fermi’s paradox

If there are so many planets out there, at least some of them should have advanced civilizations that could have colonized much of the galaxy. So what haven’t they contacted us? There are a few possibilities:

1. We are alone.2. Civilizations are common, but none have colonized

the Galaxy.3. There is a galactic civilization, but they haven’t

contacted us yet.