Download Exoplanets and Tides

Theme 4 – Exoplanets and Tides
ASTR 101
Prof. Dave Hanes
Does A Stone Fall to the Ground?
Not exactly! Remember Newton’s 3rd Law…
…and the pushup discussed earlier
Everything moves, if only just a little!
In Like Fashion
The Earth and Moon are
‘falling’ (orbiting) around
a common centre of mass:
the smaller Moon moves
a lot, the more massive
Earth less so:
Similarly the Sun and Jupiter:
Kepler Was Lucky!
Our Solar System has a very dominant central sun,
with more than 99% of the mass.
The orbiting planets have little influence on the Sun or
on each other, so the orbits are quite stable and
regular. This made Kepler’s analysis easier.
Of course, if our Solar System had been different, we
might not have evolved here to see it! Fortunately we
live on a planet with a stable orbit and climate.
One Way to Find Extrasolar Planets
As a low-mass planet swings in a big orbit, its more massive
parent star ‘wobbles’ side-to-side (which is imperceptible from
Earth!) but also towards and away from us (which is detectable,
using the Doppler shift, explained later).
http://www.physics.queensu.ca/~hanes/A101-Movies/UPS-AND.mp4
The evidence: the dark ‘absorption lines’ in the spectrum
of the star shift alternately towards the red or the blue as
the star recedes from us or is approaching.
http://www.physics.queensu.ca/~hanes/A101-Movies/EXO-DOPP.mp4
Teeter-Totters
To and Fro:
Star 51 Peg
Multiple-Planet Systems
Very complex back-and-forth
motions can be resolved to
show the existence of
multiple planets around some
stars.
A Second Detection Technique
Look for transits. Monitor a star’s brightness, looking for a
dimming that repeats perfectly periodically. (Seen from
afar, Jupiter would dim the Sun by 1% every 11.8 years.)
We don’t see the details
shown here. We merely
notice the change of
brightness. The figure
shows why that happens.
More Local Transits
Venus, an inferior planet, very rarely transits the face of the Sun
as seen from Earth. This happened most recently in 2012, as
shown here (the black dot on the Sun’s face)
This is of historical and astrophysical importance for many
reasons.
The Kepler Telescope
monitored 145,000 stars, finding many planets
http://kepler.nasa.gov
Thousands of Exoplanets Now Known!
The latest tabulation is at http://exoplanet.eu
Note that we can very rarely actually detect a
planet as a very faint dot of light beside a star.
New telescopes, especially in space, will make this
much more feasible.
Some Could be Habitable –
or Already Have Life
Note: these are not direct images of the exoplanets!
TIDES
Ocean Tides
Tidal Forces
Suppose the Earth was purely fluid, like a
sphere of water, isolated in space.
What would happen if we now put the moon
down beside it?
It Would Stretch!
The atoms closer to the moon (at A) feel a stronger
gravitational tug than those on the far side (B).
B
A
Imagine A Rocky Earth
Uniformly Covered with Water
Notice the two high tides – one pointing
towards the moon, the other away!
The Real Earth
It rotates, so it is somewhat
flattened (that is, thicker
through the equator)
Moreover, since the moon is
nearby, the oceans are
stretched out! (So too is the
rocky Earth itself, a little bit).
Two Complications
1.
1.
The oceans are not uniformly spread over the
globe. There are continents sticking up, deep
and shallow parts, etc. This complicates
things, but tides can be reliably predicted.
The Sun also raises tides. But it is at a much
greater distance (400x as far away), so its tides
are less important than the lunar tides, only
about 1/3 as high.
But They Can Co-operate!
If the Sun and Moon are lined up with the Earth
(at full moon or new moon), they work together
to raise quite high tides, called spring tides.
(Think of ‘springing up’, not the season!)
If they are at right angles, we get less dramatic
neap tides.
As Shown Here
‘Spring’ tides are higher than ‘Neap’ tides
The Long-Term Effects of Tides
Think of the rocky Earth like a ball bearing,
rotating inside an envelope of water. If there was
any friction:

The water would be carried along a bit by the rotation

and the ball bearing would gradually slow down.
Is the Earth’s rotation slowing down? Are the days
really getting longer?
Like So
This is Indeed Happening!
The Earth’s rotation is slowing down – but not
enough to affect day-to-day life.
Today is about 1/500 of a second longer than it
was a century ago.
Millions or billions of years ago, the days were
much shorter! (Earth was spinning faster.)
A Puzzle?
How can the Earth’s spinning motion dwindle? Doesn’t
angular momentum have to be conserved?
Yes it does. But as the Earth spins more slowly, the
moon is gradually moving away from us (at a rate of
~ 4 cm /year).
The moon’s altered orbital motion is where the
‘missing’ angular momentum goes.
We Measure This
bounce laser signals off lunar reflectors
Tides Have Two More
Consequences
1. Resonances
2. Heating and Disruption
I. ‘Resonances’

The Moon

Jupiter’s Galilean satellites

Pluto and Charon (its biggest moon)
The Meaning of a ‘Resonance’
Actions that are synchronized in time in some way
Like pushing a swing every time it gets back to you.
The Tidal Interactions
Between Earth and Moon
These have led, over time, to a one-to-one
resonance
That is,
 The Moon orbits us once a month, and
 it spins (rotates) on its axis once a month.
So it is constantly facing us.
This has not always been the case.
Front and Back
visible to you!
seen only by spacecraft!
The Galilean Moons of Jupiter
In 1:1 resonances with Jupiter
Imagine growing
up on the outward
side of Io – then
walking around it.
You’d see Jupiter -- 20x the size of our moon!
Not Quite ‘Perfect’
For both the Earth and Jupiter, the moons
face the planet,
-- but the planet itself spins quickly
(24 h for Earth, ~10 h for Jupiter)
So the moons always face the parent
planets, but not vice versa.
Perfect! -- Pluto and Charon
(Its Biggest Moon)
Dancing face to face!
Like So
New Horizons
The ‘New Horizons’ space probe passed within a
few thousand kilometers of Pluto in July 2015
http://www.nasa.gov/mission_pages/newhorizons/main/index.html
II. Stresses and Strains
Io – the active moon
Rings everywhere! (Complete disruption)
A recent cataclysm.
Io – the Active Moon
Furious Volcanic Activity!
Rings Everywhere
Rubble from Disrupted Satellites or Asteroids
Found around all four of the outer planets (only
Saturn’s are conspicuous)
What They Are…
(an Artist’s Impression)
Complete Disruption:
Comet Shoemaker-Levy
This comet was found as you see it here – in
pieces, having been completely disrupted by the
tidal effects of Jupiter in a recent close passage.
Astronomers realized that on a subsequent
passage, in 1994, the lumps would hit Jupiter.
The Multiple Impacts
This is an infrared picture, showing the heat
generated. (Io is the hot moon to the upper left.)
A Sobering Reminder
Each impact released enough energy to
destroy all civilization on Earth!
And this happened in recent decades, in
1994!
We will return to a discussion of such perils
later.