Discovery of Uranus' Rings

We have discussed before that all the gas planets in the solar system have rings.  Even through a small telescope Saturn has visible rings, but Jupiter, Uranus and Neptune do not. So how did astronomers discover their rings in the first place?

Hubble image of Uranus and its rings

The rings around Uranus were discovered in 1977. Astronomers knew that Uranus was going pass in front of a distant star in the night sky, from Earth's perspective. They pointed their telescopes to towards the planet each night, and expected to see the planet block the light from the star only when the star was directly behind the planet. What they actually observed was the star flickering right before and right after is passed behind the planet. This meant that there must be some unseen object near the planet blocking the starlight! The only plausible explanation was that Uranus has very thin, dim rings that are not visible from telescopes here on Earth. In 1986, Voyager flew by Uranus and imaged the rings for the first time, proving  their existence. Since then, we have discovered rings around Jupiter and Neptune in similar ways.

NASA Missions Extended

 

Artists conception of Spitzer, Planck and Kepler (left to right)

Astronomers received some great news a few days ago. Three major space telescopes, Kepler, Spitzer and Planck, have had their missions extended! This is great news, as astronomers will obtain more data and hopefully make some big discoveries! But what can we do with these telescopes?

The Kepler Space Telescope is an optical telescope has been actively searching for exoplanets. It looks at the same region of the sky 24/7, and measures the brightness of 150,000+ stars. If one of them dims for a short period of time, it might be due to a planet crossing in front of the star and blocking the light. Kepler has already found over 2000 potential exoplanets in the last 2.5 years of operation, and it's funding has been extended until 2016

The Spitzer Space Telescope is an infrared telescope that has been operating since 2004. For the telescope's detector to work properly, it needs to be kept extremely cold. Unfortunately, the cryogenics which keep it cool have run out, but the detector still functions, and some science can be done with the telescope. Astronomers have used Spitzer to look at young stars, distant galaxies, and many other objects that are "hidden" behind giant clouds of gas.  It will continue to operate for another two years.

Planck is a jointly funded NASA and ESA telescope which has been operational for about three years. It's a space based microwave/radio telescope whose main purpose is the study the cosmic microwave background. This is the first light emitted by the universe after the Big Bang. It will help us understand how the universe began by observing it right after it was born. Astronomers also use Planck to study distant galaxies, and objects in our solar system.

Image Credit:  NASA/JPL-Caltech

Finding the Planets

Today, we take a more observational approach to our astronomy lesson which will require you to go outside tonight and look at the stars. If  you've taken a look at the sky lately, you might have noticed a few extra bright objects up there. These bright objects are not really large stars, they are actually the planets in our solar system! Three of the planets (Venus, Mars and Jupiter) are visible just after sunset right now (assuming skies are clear where you are!) To find the planets, start by looking west. You should see two very bright objects in a straight line fairly low in the sky, brighter than any other stars around them. These are Venus (brightest one) and Jupiter! Once you've found them, turn around and look east. There should be another bright object in the sky that has a distinct red hue to it. That is Mars! Below are some images from Sky & Telescope magazine showing you where the planets are in relation to other stars and the moon. (Even though they say April 2nd they are about right for any day this week)

If you happen to have a telescope, or an observatory near by, take a look at these planets. If the night is very clear, you might be able to see the four Galilean moons of Jupiter or even the polar ice caps on Mars! It's really a spectacular sight!

Kepler's Third Law

The final law, Kepler's third law, is one of the most useful relations in astronomy. It states that the period of time it takes a planet to orbit the sun, squared (that's period*period), is proportional to its distance from the sun, cubed (distance*distance*distance). Or, as astronomers would say: P^2=a^3, where P is period and a is semi-major axis (i.e. distance).  The graph above shows the period and orbital distance of some planets in our solar system. The line going through all the points corresponds to the spot where P^2=a^3. The fact that all the planets fall on this line means that Kepler's third law is correct, and that we can predict the orbital time if we know the orbital distance, or vice versa. This relationship can be applied to most objects orbiting a larger object in space. Astronomers use it to estimate the period of exoplanets orbiting stars, and stars orbiting galaxy centers. 

And there you have it! Kepler's three laws of planetary motion!

Image Credit: Kevin Brown, Reflections on Relativity

Kepler's Second Law

Kepler's second law states:  The line joining the planet to the Sun sweeps out equal areas in equal intervals of time. 

 

This law is often referenced as the "law of equal areas" . So what does it mean? In the diagram above we have a planet going around the sun (or any star) following an elliptical path (as the 1st law states). When the planet is at point A, we draw an imaginary line towards the star. The planet continues to orbit the star, and lets assume one month passes. The planet is now at point B, and we draw another imaginary line towards the star. The area shaded in blue is the imaginary triangle in space that is created by the two lines we drew. We can calculate the area of this triangle because we know the length of the two lines we just drew. Now we repeat this scenario for when the planet is at points X and Y, and again it took the planet one month to go from point X to point Y. Notice that it traveled a much shorter distance on its orbit, and that the imaginary triangle we made is a lot thinner. But, again we know the length of the lines we drew, and if you calculate the area of this green triangle, you should get exactly the same amount as for the blue triangle! So in one month, the planet sweeps out a path of equal area!

Why is this the case? When the planet is closer to the star, it feels a stronger gravitational force from the star. The star sort of whips the planet around the corner closest to it, and has a weaker effect when the planet is farther away. All planets that orbit their host star in an ellipse will follow this rule.

Kepler's First Law

Johannes Kepler was a very famous astronomer. He was one of the first astronomers to understand the physics behind our solar system and how objects orbit one another. If you ever take a course in astronomy, one of the first concepts you will learn about is Kepler's three laws of planetary motion. Today I introduce Kepler's first law of planetary motion.

 

Kepler's first law: Planets orbit the sun in an ellipse with the sun at one foci .

With the technology and telescopes that we have today, it's easy to show that planets follow a squished circular shaped path around the sun called an ellipse . But how did Kepler know? Back in the early 1600's, astronomer and physicist Tycho Brahe took very precise measurements of the position of Mars in the sky. At the time, astronomers believed that all planets orbited in a circular path with the sun at the center. Assuming this is true, Brahe calculated where he expected Mars to be located in the sky throughout the year. To his surprise, the position of Mars never matched his prediction! He then gave Kepler the task of figuring out why the data and predictions did not match. Kepler discovered that if you model the Earth and Martian orbit using ellipses with the sun at one foci, Brahe's predictions would match up perfectly with his observations! We can quantify how "squished" the circle is using a parameter called eccentricity. An eccentricity of 0 means the planets path is a perfect circle. An eccentricity of 1 means that the planets path is a straight line. Planets orbit with eccentricities between 0 and 1, and most of the planets in our solar system have an eccentricity of <0.1 i.e. almost circular. This law is universal, which means it can be applied to extrasolar planetary systems as well as our own.

Van Allen Belt

Earth is surrounded by a large magnetic field caused by a molten iron core deep inside the planet. It's very similar to the bipolar magnetic field produced by a bar magnet, just on a much larger scale. The field lines extend out one pole, wrap around the earth, and re-enter at the other pole, creating a magnetic barrier around Earth. The Van Allen Belt is the part of this barrier, where most of the high energy particles aimed towards Earth are collected and safely grounded at Earth's poles. It sits about 20,000km above Earth's surface, well within the orbit of the moon. There are actually two Van Allen belts, an inner and outer one, which trap different types of particles.

This is a great thing for humans on Earth, but it poses big problems for satellites, telescopes, and space travel. Telescopes and satellites that travel through the Van Allen belt can be easily damaged by this highly concentrated radiation. This is why most satellites orbit within the belt, and most space telescopes have orbits that do not cross the belt, or cross through it once to get to a further destination.  The Van Allen belt is also a big problem for astronauts. Without special equipment, humans can not safely pass through the belt, as they would instantly be poised by the radiation. Special protective equipment from astronauts and the space shuttle was developed so that astronauts on the Apollo missions could travel safely to the moon and back.