Phys 214. Planets and Life

Dr. Cristina Buzea

Department of Physics

Room 259

E-mail: cristi@physics.queensu.ca

(Please use PHYS214 in e-mail subject)

Lecture 30. Habitability in star systems.

Extrasolar planets.

March 28th, 2008

Contents

Textbook pages 329-332, 338-353, 360-392

The evolution of surface habitability in a star system

Stars and their classification

Which stars do make good stars

Detection of extrasolar planets

The nature & evolution of habitability

A star’s habitable zone = the range of distances from the star where liquid water can be stable onthe surface of a suitable planet.

Being in a star habitable zone is NOT enough to make a world habitable. The size of the planetis very important!

E.g. The Moon is in the habitable zone of the Sun but is not habitable (too small to retain an atmospherenecessary for liquid water to be stable).

E.g. Europa is located outside the Sun’s habitable zone and yet may be habitable (because is tidally heated,allowing liquid water to exist beneath its icy surface)

The nature & evolution of habitability

Habitable zones evolve with time.

Over time the Sun’s habitable zonehas widened and moved fartherfrom the Sun.

When the Sun was younger its

habitable zone was narrower and

closer to the Sun.

In the future, the Sun’s habitable

zone will be wider and farther

from the Sun.

The end of habitability of Earth:

conservative estimates ~ 1 billionyears from now.

optimistic estimates ~ 3-4 billionyears from now.

The nature & evolution of habitability

The Sun brightens gradually with time because as H is converted into He in

the core, the number of H nuclei decreases, decreasing the fusion rate.

To maintain the balance with the gravity of the outer layers, the core

compensates by shrinking and heating up.

The outer layer surrounding the core (that still contains unburned H)

becomes very hot that ignites nuclear fusion.

The total rate of fusion is so high that the star increases in size and emits

more light.

The Sun will expand into a red giant when will run out of nuclear fuel. The

Earth surface will heat to 700oC, oceans will evaporate, runaway

greenhouse effect followed by the total loss of its atmosphere. Life not

able to survive even beneath the surface.

When the Sun ejects its

outer layer into space to

become a planetary nebula,

most likely the Earth will

be destroyed.

Surface habitability factors

Critical factors that determine the habitability of a

planet:

1) SizeE.g. Mars currently lacks surface habitability mostly because

of its small size

Large size enough to allows plate tectonics to exist.Venus (similar size to Earth) but lacks plate tectonics because

of the runaway greenhouse effect - too close to the Sun.

2) distance from parent star(the planet must be not too close or too far from its star. Must

lie within the habitable zone)

3) presence of an atmosphere

Without enough atmospheric pressure, liquid water

cannot be stable & protection against harmful

solar radiation.

To have a long lasted atmosphere, the planet must

have enough gasses trapped in interior to be

outgassed, a magnetic field, and at least a

moderate rotation.

Global warming

The recent gradual rise in the Earth’s average surface

temperature is commonly referred to as global warming.

The detailed causes of the recent warming remain an

active field of research, but the scientific consensus is

that the increase in atmospheric greenhouse gases due

to human activity caused most of the warming

observed since the start of the industrial era.

Earth - Global warming

Measurements of CO2 concentrations over the past 400,000 years show a direct correlationwith global surface temperatures.

The atmospheric CO2 concentration is higher than it has been at any time during the last400,000 years.

While there is no doubt that global temperatures are increasing, it is now becoming clearthat human activity is indeed causing global warming.

Earth - Global warming

The green stripe shows the variation between different models that take into account

both:

-natural factors (changes in sun’s brightness, effects of volcanic eruptions) &

- emission of greenhouse gases by humans.

Global warming

These changes are expressed in

terms of radiative forcing = a

measure of the influence that a

factor has in altering the

energy balance of the climate

system.

Positive forcing tends to warm the

surface while negative forcing

tends to cool it.

(checkhttp://www.physics.queensu.ca/~phys214/Links.htmClimate change 2007

Changes in the atmospheric abundance of greenhouse gases and

aerosols, in solar radiation and in land surface properties.

Aerosols

Natural and anthropogenic aerosols.

The amount of man-made aerosols isconsiderable and can induce weathercycles over populated regions, where theycan affect cloud formation.

NASA research: anthropogenic aerosolsinduce clear weekly cycles over NewYork City.

The aerosols were thickest midweek andlightest on weekends, affecting cloudsformation (J. of Geophysical Res. 110, 2005)

STRATOSPHERE

TROPOSPHERE

St. Helen, 1080

40 km plume

Solar irradiance

Some researchers estimated that the Sun

may have contributed about 25–50% of

the increase in the average global

surface temperature during 1900–2000.[Geophysical Research Letters 33 (5)]

Movie: Solar flares

Some researchers suggests that climate

models overestimate the relative effect

of greenhouse gases compared to solar

forcing and underestimate the cooling

effects of volcanic dust.

However, even with an enhanced climate

sensitivity to solar forcing, most of

the warming since the mid-20th

century is likely attributable to the

increases in greenhouse gases!!!

Stars

Finding a planet on which life might arise isselecting a sun that can provide sufficientlight and heat to support habitableworlds.

Stars spend 90% of their lives fusing hydrogeninto helium in their cores and slowlybrightening.

A star that exhausts its hydrogen core beginsto grow larger and brighter, becoming agiant or supergiant star.

Habitable planets are around stars that arein the long-lasting H-fusing stage oftheir life.

When stars finished the fusion fuel they die.

Relatively low-mass stars like our sun ejecttheir outer layers into space as planetarynebulae, leaving behind a type of dead starcalled white dwarf.

Higher mass stars die in titanic explosions –called supernovae, in which their corescollapse in either neutron stars or blackholes.

Stars

One of the fundamental principles of stellar evolution is that the

more massive a star is the faster it evolves.

A star less massive than our Sun will have a longer lifetime.

A star more massive than our Sun will have a shorter lifetime.

How do we classify stars?

Because all stars are born with basically the same composition (98% H & He), the physics

that determines the star characteristics is straightforward.

During the H burning phase, the star’s surface temperature (or spectral type) and total

luminosity is determined almost entirely by one thing – star mass!

There are seven major spectral type of stars: O, B, A, F, G, K, M.

Oh,

Be

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Fine

Girl,

Kiss

Me

The spectral sequenceOBAFGKM runs fromhot to cool in terms ofsurface temperature ofstars.

Mnemonic

Which stars do make good suns?

O-type stars (most luminous) have lifetimes too short for planet formation.

O-type stars in our galaxy are very rare (less than 1%)

B-type stars (much more luminous than our

Sun) have lifetimes long enough for planets

to form but not for life to appear.

A- and F-type stars (more luminous than the

Sun) have lifetimes long enough for planets to

form and for simple life to appear, but not long

enough for advanced life to develop.

Stars like our Sun have

lifetimes long enough

for advance life to

evolve.

K- and M-type stars (less luminous

than the Sun) have lifetimes long

enough for advanced life to evolve.

However, they might not have many

habitable planets around them

because their habitable zones are

very narrow.

Which stars do make good suns?

K- and M-type stars have lifetimes long enough for advanced life to evolve. However,

they might not have many habitable planets around them because their habitable zones

are very narrow.

Objections to their habitability:

Closer planets might be locked in a synchronous rotation, with one side facing the star.

Smaller stars have frequent and energetic flares that might negatively affect life on closer

planets.

Which stars do make good suns?

• Depending on the stellar type (mass and luminosity), habitable planets will be at

different distances from the parent star

Which stars do make good suns?

Brown dwarfs are substellar objects with insufficient mass to sustain nuclear fusion in

their cores. They have higher surface temperatures than planets and masses between 10-

to 80 times that of Jupiter. Brown dwarfs have no habitable zones because they are so

dim.

However, recent infrared observations suggest that planets are forming around them, and

one brown-dwarf has a Jupiter-size planet orbiting it.

Many brown dwarfs in constellation Orion. Infrared image of a Jupiter-size planet orbiting a brown dwarf.

Habitability of planets orbiting a multiple star system

Multiple star systems in our galaxy are fairly common, making up

around 30% of the total stars.

Stable planetary orbits:

- A large orbit around both stars in a close binary system

- An orbit close to one of the stars in a wide binary system

Detection of extrasolar planets

Two ways to search for extrasolar planets:

1) Directly – pictures or spectra of planets

2) Indirectly – by measuring stellar properties

(position, brightness, spectra)

• astrometric technique

• Doppler technique

• transit technique

• Gravitational lensing

The few extrasolar planets

that have been detected

directly to date are very

large and at great

distances from their

parent star.

Detection of extrasolar planets - Astrometry

Astrometry uses regular changes in the

positions of the parent stars with respect to

more distant stars as they move across the

sky to detect extrasolar planets around

other star systems.

Most extrasolar planets have been detected by

observing the gravitational tug they exert

on the stars they orbit.

All the objects in a star system, including the

star, orbit the system’s center of mass.

The center of mass of the solar system is close to

center but not exactly at the center of the Sun.Orbital path of the Sun around the center of mass of the

Solar System as it would appear from a distance of 30

light-years away. 1960-2025

Detection of extrasolar planets - Doppler technique

Detection of Doppler shifts in the spectra of the parent stars has been the MOST successful in

detecting extrasolar planets around other star systems.

Stars exhibit Doppler shift only if they are moving toward or away from us along the line of sight.

The wavelengths of radiation from a star that is moving toward us are shorter.

The wavelengths of radiation from a star that is moving away from us are longer.

Detection of extrasolar planets - Doppler technique

The radial velocity curve of a star with an extrasolar planet is a plot of radial velocity againsttime.

If a star has an extrasolar planet

- the amplitude of its radial velocity curve is related to the planet’s mass.

- the wavelength of its radial velocity curve is related to the planet’s orbital period.

- the symmetry of its radial velocity curve

is related to the planet’s orbital shape.

(The uneven nature of the change in velocity

indicates that the planet is in a highly elliptical orbit.)

If a star has a high mass planetat a small distance form it, itsradial velocity curve wouldhave a large amplitude, shortwavelength.

Massive planets closed to theirparent star -easiest to detectusing the Doppler shift.

A low mass planet far from itsparent star would be the mostdifficult to detect using theDoppler shift method.

Detection of extrasolar planets - Doppler technique

When we measure the mass of a planet using the Doppler shift method, we know that it is

mass could well be larger. The Doppler shift method of detecting extrasolar planets only

give us the minimum mass of a planet because we don’t necessarily know the angle the

planet’s orbit makes with our line of sight.

Most of the extrasolar planets detected to date are found very close to the parent stars.

If we view a planetary orbit face-on, we will not detect any

Doppler shift at all.

We can detect Doppler shift only if the planet and star have

a part of their orbital velocities directed toward or away

from us.

Detection of extrasolar planets - Transit technique

The transit method for detecting extrasolar

planets is based on detection of brightness

changes in a star as a planet passes in front of

it. These changes depend on the planet size.

When Mercury or Venus passes in front of the

disk of the Sun, we call this a transit.

For the transit of an extrasolar planet to be

observed, the orbital plane of the planet has

to be aligned along our line of sight.

This method detects planets orbiting closed to

their star.

Detection of extrasolar planets - Transit technique

•Hubble Space Telescope field of view in the Sagittarius Window Eclipsing Extrasolar Planet Search

(SWEEPS).

•Half of these stars are bright enough for Hubble to monitor for any small, brief and periodic dips in

brightness caused by the passage of an exoplanet passing in front of the star.

•The green circles identify 9 stars that are orbited by planets with periods of a few days. Planets so

close to their stars with such short orbital periods are called "hot Jupiters."

Detection of extrasolar planets - Gravitational lensing

Gravitational lensing is the process by which a massive object magnifies and distorts

the light from an object behind it. Mass distorts space -> light rays bend slightly

when passing near objects with large mass.

Gravitational lensing detection has discovered the lowest-mass planet to date – only 5

times the mass of earth planets in large orbits.

Properties of extrasolar planet discovered so far

Hot-jovian describes the most common type of extrasolar planet discovered to date.

The orbits of most extrasolar planets detected to date are highly elliptical.

Most of the extrasolar planetary systems discovered to date are very different than our

own solar system having Jovian-sized planets close to their parent stars.

Properties of extrasolar planet discovered so far

The existence of giant planets in sub-Mercurian orbits and in excentric orbits has come as a

surprise and is forcing theorists to revise their understanding of how young planetary

systems evolve.

According to our current theory of planet formation, Jupiter-like planet cannot form close to

its parent star because it would be too hot for gases to condense.

However, they can form farther out and then migrate inward.

The inward migration of a Jovian-like planet in an extrasolar planetary system will alter

the probability of life appearing on inner terrestrial planets - it would greatly decrease

the chance because the orbits of the inner, terrestrial-like planets would be disrupted

How to detect life on extrasolar planets

Spectra from different Earth-like planets

The simplest way we might detect life on an extrasolar planet isto analyze the spectrum of reflected light from the planet.

Galactic constraints

Both the amount of heavy elements AND the amount of radiation from the center determine

the position of a galaxy’s Galactic Habitable Zone.

It is difficult for planets with life to form around Sun-like stars in the inner parts of the disk

of our galaxy because there would be too much harmful radiation

It is difficult for Earth-like planets to form around Sun-like stars in the outer parts of the

disk of our galaxy because there would be insufficient amounts of heavy elements.

Green – habitable zone

Impact rates and Jupiter

If our solar system hadn’t formed with a Jupiter-sized planet, the rate of impacts on the

Earth may have been much higher, possibly preventing the appearance of life.

Next lecture

• The search for extraterrestrial intelligence

• The Drake equation

• Interstellar travel

• Conclusions

• The final review lecture to be posted online later

today!