18.3 Life Around Other Stars

Our goals for learning• Are habitable planets likely?• Are Earth-like planets rare or common?

Are habitable planets likely?

Caveat: with telescopes we can search other solar systems only for planets with habitable surfaces — not for worlds with Europa-like subsurface oceans.

Definition:

A habitable world contains the basic necessities for life as we know it, including liquid water.

• It does not necessarily have life.

At least 3 constraints on habitable star systems:

1) Old enough to allow time for evolution (rules out high-mass stars - 1%)

2) Need to have stable orbits (might rule out binary/multiple star systems - 50%)

Figure 18.10Binary-Star Planets

At least 3 constraints on habitable star systems:

1) Old enough to allow time for evolution (rules out high-mass stars - 1%)

2) Need to have stable orbits (might rule out binary/multiple star systems - 50%)

3) Size of “habitable zone”: region in which a planet of the right size could have liquid water on its surface.

Even so… billions of stars in the Milky Way seem at least to offer the possibility of habitable worlds.

The more massive the star, the larger the habitable zone: higher probability per star of a planet in this zone (but low-mass stars most common, so majority of habitable planets may orbit around them).

Elements and Habitability• Some scientists argue that

proportions of heavy elements, and amounts of ultraviolet radiation, need to be just right for the formation of habitable planets.

• If so, then Earth-like planets are restricted to a galactic habitable zone.

Impacts and Habitability• Some scientists argue

that Jovian planets are necessary to reduce the rate of asteroid & comet impacts on other planets in the system.

• If so, then Earth-like planets are restricted to star systems with Jovian planets.

Climate and Habitability• Some scientists argue

that plate tectonics and/or a large Moon are necessary to keep the climate of an Earth-like planet stable enough for life.

Are Earth-like planets rare or common?

• Galactic “habitable zone”: minimum limits on heavy element abundance, distance from galactic center?

• Jovian protection from frequent impacts?

• Climate stabilized by a large Moon and plate tectonics?

We don’t yet know how important or negligible these concerns are.

How to find planets

• Doppler shift of star

• Transit of planet in front of star

• Astrometry (star’s motion on the sky)

• Microlensing (gravitational lensing of a background star by a foreground star+planet)

• Direct imaging, from the ground or from space (Hubble now, Webb later)

The First Images of Extrasolar Planets

• Images taken in the last few years show planets orbiting bright, young, nearby stars or brown dwarfs

• Only a few of the >400 planets known to orbit other stars have been imaged

Hubble Space Telescope visible image of the star Fomalhaut (whose light was blocked), with a dust belt similar to the Kuiper belt. Inset: Images taken ~2 years apart show a planet moving around the star.

Star location

Neptune-sizedorbit

One planet orbiting Fomalhaut

Glowing Young Planets• This star has three orbiting

planets - the first imaged exoplanetary system!

• Planets are much fainter than their parent star, so are difficult to image

• Why are these pictures possible?

• Advanced observing techniques were used to block the star’s light

• Observations were repeated over years, confirming planetary motion

• These massive, large planets are young and hot, and therefore glow more brightly than by reflected starlight alone

Keck Observatory infrared image of star HR8799 and three orbiting planets with orbital directions indicated by arrows. The light from the star was subtracted, but a lot of ‘noise’ remains.

Three planets orbiting HR8799

Finding Earth-like planets will be hard

• Looking for an Earthlike planet around a nearby star is like standing in Toronto and looking for a pinhead in British Columbia — with a very bright light bulb right near it.

• But new technologies in the next decade or two should help…

• Kepler will monitor 100,000 stars for transit events for 4+ years (3 years so far).

• COROT is monitoring a similar number of brighter stars for 6+ years (5 so far)

• Earth-mass planets soon?

Later: SIM (2020s?), TPF-I (2030s?): interferometers to obtain optical/infrared spectra and crude (few-pixel) images of Earth-size planets.

Spectral signatures of life

oxygen/ozone

Earth

Venus

Mars

What have we learned?• Are habitable planets likely?• Billions of stars have at least moderate size habitable

zones in which life bearing planets might exist. We do not yet have the technology to search for habitable planets directly, but several planned missions should be able to begin the search soon.

What have we learned?

• Are Earth-like planets rare or common?

• We don’t know. Arguments can be made on both sides of the question, and we lack the data to determine their validity at present.

18.4 The Search for Extraterrestrial Intelligence

Our goals for learning• How many civilizations are out there?• How does SETI work?

Figure 18.7Drake Equation

The Drake EquationNumber of civilizations with whom we could

potentially communicate

= N fHP flife fciv fnow

N = total # of stars in Galaxy with habitable zones

fHP = average number of habitable planets per star

flife = fraction of habitable planets with life

fciv = fraction of life-bearing planets w/ civilization at any time in past 10 billion years

fnow = fraction of civilizations around now = lifetime of typ. technological civilization / 10 billion years

We do not know the values for the Drake Equation, but we can make educated guesses that improve with time.

N : probably tens (if not hundreds) of billions

fHP : ??? Many stars have planets, but in zone?

flife : ??? Hard to say (near 0 or near 1)

fciv : ??? It took 4 billion years on Earth (Fig.)

fnow : ??? Can civilizations survive long-term?

Let’s make some guesses about these values.

fciv : Are we “off the chart” smart?

If there are N=100 billion (1011) habitable stars in our galaxy, how many technological civilizations do you

estimate currently exist in our Galaxy?(# = N fHP flife fciv fnow)

1. Less than 12. 1 to 103. 11 to 1004. 101 to 10005. 1001 to 10,0006. 10,001 to 100,0007. 100,001 to 1,000,0008. More than 1,000,000