How can the sun have a strong gravitational pull if it's made up of gases and not a solid mass like the earth?
The gravitational field of an object is determined solely by:
1) how massive it is, and 2) how far away from it you are
Whether it's a gas or a liquid or a solid doesn't make any difference. The Sun is a lot more massive than the Earth, and so it has a stronger gravitational field. From our everyday experience, it might not be so obvious that a gas has mass, but it's true. The Sun exerts the same gravitational force on the planets as it would if it had the same mass but was made of rock.
This page was last updated June 28, 2015.
Chris studies the large scale structure of the universe using the peculiar velocities of galaxies. He got his PhD from Cornell in 2005, and is now a Research Assistant Professor at the University of Western Australia.
Gravity is the force by which a planet or other body draws objects toward its center. The force of gravity keeps all of the planets in orbit around the sun.
What else does gravity do?
Why do you land on the ground when you jump up instead of floating off into space? Why do things fall down when you throw them or drop them? The answer is gravity: an invisible force that pulls objects toward each other. Earth's gravity is what keeps you on the ground and what makes things fall.
An animation of gravity at work. Albert Einstein described gravity as a curve in space that wraps around an object—such as a star or a planet. If another object is nearby, it is pulled into the curve. Image credit: NASA
Anything that has mass also has gravity. Objects with more mass have more gravity. Gravity also gets weaker with distance. So, the closer objects are to each other, the stronger their gravitational pull is.
Earth's gravity comes from all its mass. All its mass makes a combined gravitational pull on all the mass in your body. That's what gives you weight. And if you were on a planet with less mass than Earth, you would weigh less than you do here.
Image credit: NASA
You exert the same gravitational force on Earth that it does on you. But because Earth is so much more massive than you, your force doesn’t really have an effect on our planet.
Gravity in our universe
Gravity is what holds the planets in orbit around the sun and what keeps the moon in orbit around Earth. The gravitational pull of the moon pulls the seas towards it, causing the ocean tides. Gravity creates stars and planets by pulling together the material from which they are made.
Gravity not only pulls on mass but also on light. Albert Einstein discovered this principle. If you shine a flashlight upwards, the light will grow imperceptibly redder as gravity pulls it. You can't see the change with your eyes, but scientists can measure it.
Black holes pack so much mass into such a small volume that their gravity is strong enough to keep anything, even light, from escaping.
Gravity on Earth
Gravity is very important to us. We could not live on Earth without it. The sun's gravity keeps Earth in orbit around it, keeping us at a comfortable distance to enjoy the sun's light and warmth. It holds down our atmosphere and the air we need to breathe. Gravity is what holds our world together.
However, gravity isn’t the same everywhere on Earth. Gravity is slightly stronger over places with more mass underground than over places with less mass. NASA uses two spacecraft to measure these variations in Earth’s gravity. These spacecraft are part of the Gravity Recovery and Climate Experiment (GRACE) mission.
The GRACE mission helps scientists to create maps of gravity variations on Earth. Areas in blue have slightly weaker gravity and areas in red have slightly stronger gravity. Image credit: NASA/University of Texas Center for Space Research
GRACE detects tiny changes in gravity over time. These changes have revealed important details about our planet. For example, GRACE monitors changes in sea level and can detect changes in Earth’s crust brought on by earthquakes.
The Sun is keeping you close. After all, you are orbiting it just like the Earth. You don't fly off into space because the Earth and you experience the same acceleration due to the Sun's gravitational force, so you orbit together; this is sometimes called the equivalence principle.
If, however, you were floating near Earth but closer to the Sun, you would experience stronger gravity. You would be in a smaller orbit which would make you drift away from the Earth. You wouldn't fall into the Sun, though.
Edit: I forgot to say something about the outer planets, something which the other answers touch on but I think get wrong. First, we should speak of acceleration rather than force, because like I said earlier all objects at a given distance from the Sun experience different forces but the same acceleration.
You ask "how come the Sun is strong enough to keep the distant planets in orbit but I don't fall into it?". The important point is that you don't need such a huge acceleration to keep the planets in orbit, because they are far away and move very slowly.
But, the smallness of the acceleration isn't the reason you don't feel it. The reason is that you're in free fall around the Sun; even if you were zipping around kilometers from the Sun's surface, you would not feel the huge gravitational force, because it affects everything around you in exactly the same way (disregarding tidal effects).