How do we know how much a planet weighs?

It is common that in the news about new exoplanets, we find things like: « astronomers discover a planet that is five times heavier than Jupiter. »

How can we know how much something so far weighs? To begin with, how can we know how much Jupiter or Earth weighs?

Weight and mass

On a daily basis we ask ourselves « how much do you weigh? » And we give an answer in kilograms.

That means that we are actually talking about the mass: the amount of matter in something, or in a person.

The weight on the other hand, it is the force with which the Earth attracts objects due to the gravity.

Mass is related to weight, because that force of attraction will depend on the mass of the object.

Isaac Newton was the one who first described the characteristics of this gravitational attraction, in his book « Philosophiæ naturalis principia mathematica«  (Mathematical principles of natural philosophy), published in 1687.

Apples and planets

When Newton published his book, Physics was already a science in its own right for years, but without a doubt that work determined a new way of understanding and explaining the world.

It includes the famous « Newton’s three laws », about motion and also the law of universal gravitation and the calculation– A whole new branch of mathematics that Newton developed to describe his laws.

And it turns out that as the law of universal gravitation It not only explains how apples fall on scientists’ heads, but how planets rotate, it also gives us the key to knowing how much planets weigh. Rather its mass.

What Newton found is that the gravitational attraction between two objects depends on the amount of matter they have and the distance at which they are: the more mass they have, the more they attract, the closer they are, the more they attract.

Stars and satellites

Determining the size of a star or planet is not that complicated, we can do it with astronomical observations, but for the mass we will have to take into account the gravitational attraction.

Since gravity always happens in pairs, to determine the mass of a planet, we will have to olook at the way it orbits its star or how it interacts with its natural satellites.

For example: the Earth and the Moon attract each other, with a force that depends on its masses and that is something that is reflected in the way the Moon orbits around us.

What we do is observe those gravitational « jerks » that occur between celestial objects and then use the equations of motion, that Newton wrote more than three centuries ago, to know the mass of some planet, or satellite, or star.

The further away the planet is from which we want to know its mass, the observation of the gravitational interactions it will be more complicated.

To overcome these difficulties, devices such as NEID, a NASA spectrograph, which is capable of observing slight gravitational fluctuations between stars and planets, which will allow us to continue “weighing” the Universe, without scales and without leaving home.