How many stars and planets exist in the universe?

Aug 18, 2017

There is no good answer for either.

Explanation:

First of all, the existence of exoplanets is very new and I believe the count right now is in the thousands and climbing. All such planets are strictly within our galaxy.

As to stars, again, it is anybody's guess. There are about 100 billion galaxies but astronomers expect this number to double as technology improves. Each galaxy, depending upon size, contains billions of stars ranging from a few billion to 300 billion or more. The real difficulty in star counting is that so many galaxies are so far away astronomers are left to both guess their size and star count.

Aug 19, 2017

We can only estimate the number of stars and planets in the universe. A figure of ${10}^{24}$ stars and a similar number of planets is a reasonable estimate.

Explanation:

The universe is so large that we can't count the number of stars. Most are too far away that we can't see them.

We can estimate the number of stars in our galaxy. Even this is hard because the galactic centre is so bright and full of stars that we can't see many stars in the galactic centre and we can't see the stars on the other side of the galactic centre to us. We can estimate the number of stars we can see and extrapolate. This gives an estimate of ${10}^{12}$ stars in our galaxy.

Next we need to estimate the number of galaxies. Again a figure of ${10}^{12}$ galaxies is reasonable.

This gives a total order of magnitude of ${10}^{24}$ stars in the universe.

Estimating the number of planets is even more difficult. It is only recently that we have been able to detect planets around other stars. About 3,500 planets are know to be orbiting other stars.

We are staring to learn that planets are common. It is a reasonable assumption to make that most stars have planets. We can therefore estimate that the number of planets is a similar order of magnitude to the number of stars. Namely ${10}^{24}$.