Chapter 15 · 15.1 Modeling Planet Formation 15.2 Solar System Regularities and Irregularities 15.3...

Lecture Outlines

Astronomy Today

8th Edition

Chaisson/McMillan

© 2014 Pearson Education, Inc.

Chapter 15


Chapter 15

Exoplanets


15.1 Modeling Planet Formation

15.2 Solar System Regularities and Irregularities

15.3 Searching for Extrasolar Planets

15.4 Exoplanet Properties

Discovery 15-1 The Closest Exoplanet

15.5 Is Our Solar System Unusual?

Units of Chapter 15


We now have many

planetary systems other

than our own to test

formation theories – 900

confirmed exoplanets and

2500 candidates as of mid-

2013

15.1 Modeling Planet

Formation


Review of condensation theory:

• Large interstellar cloud of gas

and dust starts to contract,

heating as it does so

• Sun forms in center; dust

provides condensation nuclei,

around which planets form

• As planets grow, they sweep

up smaller debris near them

15.1 Modeling Planet

Formation


Condensation theory covers the 10 points mentioned at

the beginning.

What about the exceptions?

1. Two large bodies may have merged to form Venus.

2. Earth–Moon system may have formed after a collision.

15.2 Solar System Regularities

and Irregularities


3. Late collision may have caused Mars’s north–south

asymmetry and stripped most of its atmosphere.

4. Uranus’s tilted axis may be the result of a glancing collision.

5. Miranda may have been almost destroyed in a collision.

6. Interactions between jovian protoplanets and planetesimals

could be responsible for irregular moons.

7. Binary Kuiper belt objects (including the Pluto-Charon

system) could have formed through collisions before ejection

by interactions with the jovian planets.


and Irregularities (cont.)


Many of these explanations have one thing in

common—a catastrophic, or near-catastrophic,

collision at a critical time during formation.

Normally, one does not like to explain things by

calling on one-time events, but it is clear that the

early solar system involved almost constant

collisions. Some must have been exceptionally large.


and Irregularities (cont.)


15.3 Searching for Extrasolar

PlanetsMost extrasolar planets have been discovered

indirectly, through their gravitational or optical effects,

and they cannot be seen directly due to the glare of

their star. However, a few dozen exoplanets have

indeed been detected this way.


Many planets around other stars have been detected

because they are large enough to cause the star to “wobble”

as the planet and star orbit around their common center of

mass.


Planets


If the “wobble” is transverse

to our line of sight, it can

also be detected through the

Doppler shift as the star's

motion changes.


Planets



Planets

An extrasolar planet may

also be detected if its orbit

lies in the plane of the line

of sight to us. The planet

will then eclipse the star,

and if the planet is large

enough, some decrease in

luminosity may be

observed.


More than 900 extrasolar planets have been

discovered so far, with about 2700 more

candidates waiting to be confirmed:

• Most are in the “cold Jupiter” or “cold Neptune”

category due to size and distance from parent star

• Orbits are generally somewhat smaller than the

orbit of Jupiter

• Most orbits have high eccentricity




The upper plot shows masses

and orbital semimajor axes for

hundreds of knows extrasolar

planets, with Jupiter, Neptune,

and Earth for comparison.

The lower shows planetary radii

and orbital semimajor axes for

thousands of exoplanet

candidates.


Orbits of many of the known extrasolar planets. Note that

some of them are very close to their star:



Planets orbiting within 0.15 AU of their stars are called “hot

Jupiters”; they are not included in the previous figure but are

numerous.

Stars with composition like our Sun are much more likely to

have planets, showing that the “dusty disk” theory is

plausible.

Some of these “planets” may actually be brown dwarfs, but

probably not many.



Recently, a planet with a mass close to the mass of the

Earth has been discovered orbiting our closest neighbor

star system, Alpha Centauri. But it is about 25 times closer

to its parent stars (Alpha Centauri A and B) than the Earth is

to our Sun. The figures below show the Alpha Centauri

system (left) and an artist’s conception of this planet (right).

Discovery 15-1 The Closest

Exoplanet


This figure shows two transiting super-Earths plus the nine

“habitable” exo-Earths (as of mid-2013), compared to Earth

and Neptune.



The other planetary systems discovered so far

appear to be very different from our own.

Selection effect biases sample toward massive

planets orbiting close to parent star; lower-mass

planets cannot be detected this way.

15.5 Is Our Solar System

Unusual?


This is an example of a “cold Jupiter” in another system. Its

orbit is very similar to that of Jupiter’s (blue). Also included is

an artist’s conception of such a planet.


Unusual?


Current theories include the

possibility that Jupiter-like

planets could migrate

inward, through friction with

the solar nebula


Unusual?


A number of Earthlike planets have now been observed,

although due to detection difficulties most exoplanets still fall

into the “hot Jupiter” category, making other planetary

systems look quite different from our own.

Until we are able to observe much smaller planets at much

larger distances from their parent stars, we will not know just

how unusual our own system is – or if it is unusual at all.


Unusual?


This figure shows the size of

the habitable zone – where

there is a possibility of liquid

water being present – as a

function of the mass of the

parent star. Note that the

presence or absence of a

greenhouse effect (runaway or

otherwise) can affect the

surface temperature of a planet

considerably.


Unusual?


• Condensation theory leads us to expect that other systems

will be coplanar with planets orbiting in the same sense.

• Random collisions will lead to irregular properties.

• Most extrasolar planets have been discovered through

wobbling of parent stars, or through transits.

• There are 900 planets and 2500 candidates so far.

• 20% of systems, and 33% of candidates, have multiple

planets per system.

• 20 Earths and super-Earths are in habitable zones.

• We don’t yet have enough information to tell how unusual

our own system is.

Summary of Chapter 15

Chapter 15 · 15.1 Modeling Planet Formation 15.2 Solar System Regularities and Irregularities 15.3...

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Transcript of Chapter 15 · 15.1 Modeling Planet Formation 15.2 Solar System Regularities and Irregularities 15.3...