One way to make sense of the gravitational interaction between a planet and a star is to imagine a game of tug-of-war. On one side, you have the star - a massive object with a really powerful gravitational field.
On the other side, you have the planet, much smaller, with a whole lot less gravity.
We know who wins this game - the star. That's why planets orbit stars and not the other way around.
But even though the planet is small, it still has some gravitational force. It still has an effect on its host star, even if that effect is much less pronounced than the one the star has on the planet.
- but two can play at the gravity game
Take a look at the above animation. At first glance, things look normal. There's a big star and a small planet, and the small planet orbits the big star. You've probably seen this many times.
But check out the star. See how it's moving a little bit, too? The effect is exaggerated for this animation, but that's what actually happens in space. The planet's gravity causes the star to 'wobble'around a little bit.
As you might imagine, the bigger the planet, the bigger the effect it has on its star. Small planets, like Earth, make their stars only wobble a tiny bit. Bigger planets, like Jupiter, have a much stronger effect.
A star's 'wobble' can tell us if a star has planets, how many there are, and how big they are.
Wobbling stars are great for finding exoplanets, but how do we see the wobbling stars?
The method used is one called 'Doppler shift'. It's named after the physicist who figured it out about 150 years ago.
Energy - sound, radio waves, heat, and light - moves in waves. Like the waves you see in the animation above.
Those waves can be stretched and squeezed, based on the movement of the object that's producing them.
You may not know it, but you've probably experienced the Doppler effect before. Have you ever noticed how the sound of an ambulance passing you on the street gets higher in pitch as it gets close to you, and then lower in pitch as it speeds away?
The reason is because when an object that emits energy (like an ambulance speaker or a massive, burning star) moves closer to you, the waves bunch up and squish together. And when the object is moving away, the waves stretch out.
Those changes in the wavelength change how we perceive the energy that we're seeing or listening to. As sound waves scrunch together, they sound higher in pitch. And when visible light waves scrunch together, they look more blue in color.
When sound waves stretch out, they sound lower in pitch. And when visible light waves stretch out, they make an object look more reddish.
This change in color is called 'redshift', and scientists can use it to see if an object in the sky is moving towards us or farther away.
You can see this method working in the animation above. The planet causes the star to wobble around in its orbit, and as the planet moves to and fro, the light waves compress together and then stretch out, changing the color of the light we see.
The radial velocity method was one of the first successful ways to find exoplanets, and continues to be one of the most productive methods. Often, this method will be used to confirm planets found with other methods - an extra step that can prove a planet exists.
Lots of astronomers and telescopes around the world use this method to discover exoplanets, but two notable observatories where this work happens are the Keck Telescopes in Hawaii and the La Silla Observatory in Chile.
A solar eclipse is one of the coolest astronomical events you'll ever experience. It happens when the moon passes directly in front of the sun, blocking its light.
This is similar to how the transit method finds exoplanets. When a planet passes directly between an observer and the star it orbits, it blocks some of that star's light. For a brief period of time, that star actually gets dimmer. It's a tiny change, but it's enough to clue astronomers into the presence of an exoplanet around a distant star.
The graph you see being drawn on the left side of the animation is what astronomers call a 'light curve'. It's a chart of the level of light being observed from the star. When a planet passes in front of the star and blocks some of its light, the light curve indicates this drop in brightness.
The size and length of a transit can tell us a lot about the planet that's causing the transit. Bigger planets block more light, so they create deeper light curves. You can see that in the animation above, when you click 'different planet sizes and distances'.
Also, the farther away a planet is, the longer it takes to orbit and pass in front of its star. So the longer a transit event lasts, the farther away that planet is from its star.
When you click on 'multiple planets' you'll see that light curves get complicated when more planets are transiting a star. The combined light curves can give us the same information as a single one, it just takes more work from astronomers to pick out each planet in the data.
The transit method isn't just useful for finding planets, it can also give us information about the composition of a planet's atmosphere or its temperature.
When an exoplanet passes in front of its star, some of the starlight passes through its atmosphere. Scientists can analyze the colors of this light in order to get valuable clues about its composition. Using this method, they've found everything from methane to water vapor on other planets.
The transit method has been spectacularly successful at finding new exoplanets. NASA's Kepler mission, which hunted for planets using the transit method from 2009 - 2013, found thousands of possible exoplanet discoveries and gave astronomers valuable information about the distribution of exoplanets in the galaxy.
Exoplanets are far away, and they are millions of times dimmer than the stars they orbit. So, unsurprisingly, taking pictures of them the same way you'd take pictures of, say Jupiter or Venus, is exceedingly hard.
New techniques and rapidly-advancing technology are making it happen.
The major problem astronomers face in trying to directly image exoplanets is that the stars they orbit are millions of times brighter than their planets. Any light reflected off of the planet or heat radiation from the planet itself is drowned out by the massive amounts of radiation coming from its host star. It's like trying to find a flea in a lightbulb, or a firefly flitting around a spotlight.
On a bright day, you might use a pair of sunglasses, or a car's sun visor, or maybe just your hand to block the glare of the sun so that you can see other things.
This is the same principle behind the instruments designed to directly image exoplanets. They use various techniques to block out the light of stars that might have planets orbiting them. Once the glare of the star is reduced, they can get a better look at objects around the star that might be exoplanets.
There are two main methods astronomers use to block the light of a star.
One, called coronography, uses a device inside a telescope to block light from a star before it reaches the telescope's detector. Coronagraphs are built as internal add-ons to telescopes, and are now being used to directly image exoplanets from ground-based observatories.
Another method is to use a 'starshade', a device that's positioned to block light from a star before it even enters a telescope. For a space-based telescope looking for exoplanets, a starshade would be a separate spacecraft, designed to position itself at just the right distance and angle to block starlight from the star astronomers were observing.
Direct imaging is still in its beginning stages as an exoplanet-finding method, but there are high hopes that it will eventually be a key tool for finding and characterizing exoplanets. Future direct-imaging instruments might be able to take photos of exoplanets that would allow us to identify atmospheric patterns, oceans, and landmasses.
Among his many insights, Albert Einstein rethought the concept of gravity, defining it less as a mysterious attraction between objects and more as a geometric property of spacetime.
In other words, big objects warp the fabric of space. This effect causes light to distort and change direction when affected by the gravity of a massive object, like a star or a planet.
This change of direction can cause some pretty interesting things to happen. Sometimes, gravity can bend and focus light like a lens in a magnifying glass or pair of glasses.
Gravitational microlensing happens when a star or planet's gravity focuses the light of another, more distant star, in a way that makes it temporarily seem brighter.
In the animation above, you can see the rays of light from the more distant star bend around the exoplanet and then the exoplanet's star. In the same way that a magnifying glass can focus the sun's light onto a tiny, very bright spot on a piece of paper, the gravity of the planet and the star focus the light rays of the distant star onto the observer.
The graph on the left indicates the changing brightness of the distant star as its light is lensed and focused onto the observer. The star starts to get brighter, then there's a brief blip of brightness from the lensing action of the planet.
The light levels fall after the planet is lensed but they continue to increase because of the continued lensing action of the star. Once the lensing star moves out of the optimum position, the brightness of the more distant star fades away.
To an astronomer, a lensing event looks like a distant star that gets gradually brighter over the space of a month or so, then fades away. If a planet happens to be lensed, it looks like a brief blip of light that happens during this brightening and dimming process.
Astronomers can't predict when or where these lensing events will happen. So they have to watch large parts of the sky over a long period of time. When they record a star getting brighter and then dimming in the pattern of lensing objects, they analyze the data to get information about the estimated size of the star.
Sometimes, free-floating planets in space, ones that don't orbit a star, will cause quick microlensing events that astronomers will record. These events give us an idea of how common these so-called 'rogue' planets are in the galaxy. The animation on the right shows what that kind of event looks like to a telescope.
First, go and read the Radial Velocity text for an explanation of how planets cause their stars to wobble around in space. We'll wait!
Okay, back? Good.
Doppler shifts aren't the only way astronomers can find stars that are wobbling due to the gravity of their planets. The wobble can also be visible as changes in the star's apparent position in the sky.
In other words, scientists can actually detect the star's position wiggling around in space.
Astrometry, as this method is called, is still amazingly hard to do. Stars wobble such a minute distance that it's very difficult to accurately detect the wobble from planets, especially small ones the size of Earth.
In order to track the movement of these stars, scientists take a series of images of a star and some of the other stars that are near it in the sky. In each picture, they compare the distances between these reference stars and the star they're checking for exoplanets.
If the target star has moved in relation to the other stars, astronomers can analyze that movement for signs of exoplanets.
Astrometry requires extremely precise optics, and is especially hard to do from the Earth's surface because our atmosphere distorts and bends light.