Theme 4 – Exoplanets and Tides

ASTR 101Prof. 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!

Teeter-Totters

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.

How 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.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/UPS-AND.mp4

The evidence: the dark ‘absorption lines’ in the spectrum

of the star shift alternately towards the red or the blue asthe star recedes from us or is approaching.

http://www.astro.queensu.ca/~hanes/ASTR101-Fall2015/ANIMS/EXO-DOPP.mp4

To and Fro: Star 51 Peg

Multiple-Planet Systems

Very complex back-and-forth motions can be resolved toshow 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 detailsshown here. We merely notice the change ofbrightness. The figureshows 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. 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.

2. The Sun also raises tides. But its larger distance (400x as far away) makes these 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

wasany 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 – thenwalking 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 EverywhereRubble 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 – inpieces, having been completely disrupted by

thetidal 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.