There are different models for estimating the number of stars in the Milky Way and the answers they give differ depending on what is used as the average mass of a star. The most common answer seems to be that there are 100 billion stars in the Milky Way on the low end and 400 billion on the high end.
But, What is Milky Way?
The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy’s appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye.
The term Milky Way is a translation of the Latin via lactea, from the Greek γαλακτικός κύκλος (galaktikos kýklos), meaning “milky circle.” From Earth, the Milky Way appears as a band because its disk-shaped structure is viewed from within. Galileo Galilei first resolved the band of light into individual stars with his telescope in 1610.
Until the early 1920s, most astronomers thought that the Milky Way contained all the stars in the Universe. Following the 1920 Great Debate between the astronomers Harlow Shapley and Heber Curtis, observations by Edwin Hubble showed that the Milky Way is just one of many galaxies.
The Milky Way is a barred spiral galaxy with an estimated visible diameter of 100,000–200,000 light-years, but only about 1000 light-years thick at the spiral arms (more at the bulge). Recent simulations suggest that a dark matter area, also containing some visible stars, may extend up to a diameter of almost 2 million light-years.
The Milky Way has several satellite galaxies and is part of the Local Group of galaxies, which form part of the Virgo Supercluster, which is itself a component of the Laniakea Supercluster.
It is estimated to contain 100–400 billion stars and at least that number of planets. The Solar System is located at a radius of about 27,000 light-years from the Galactic Center, on the inner edge of the Orion Arm, one of the spiral-shaped concentrations of gas and dust.
The stars in the innermost 10,000 light-years form a bulge and one or more bars that radiate from the bulge. The galactic center is an intense radio source known as Sagittarius A*, a supermassive black hole of 4.100 (± 0.034) million solar masses. Stars and gases at a wide range of distances from the Galactic Center orbit at approximately 220 kilometers per second.
The constant rotational speed appears to contradict the laws of Keplerian dynamics and suggests that much (about 90%) of the mass of the Milky Way is invisible to telescopes, neither emitting nor absorbing electromagnetic radiation. This conjectural mass has been termed “dark matter”. The rotational period is about 240 million years at the radius of the Sun.
The Milky Way as a whole is moving at a velocity of approximately 600 km per second with respect to extragalactic frames of reference. The oldest stars in the Milky Way are nearly as old as the Universe itself and thus probably formed shortly after the Dark Ages of the Big Bang.
On 12 May 2022, astronomers announced the image, for the first time, of Sagittarius A*, the supermassive black hole at the center of the Milky Way galaxy.
How Big Is the Milky Way?
Our galaxy is a gravitationally bound collection of stars, swirling in a spiral through space. Based on the deepest images obtained so far, it’s one of about 2 trillion galaxies in the observable universe.
Groups of them are bound into clusters of galaxies, and these into superclusters; the superclusters are arranged in immense sheets stretching across the universe, interspersed with dark voids and lending the whole a kind of spiderweb structure.
Our galaxy probably contains 100 to 400 billion stars and is about 100,000 light-years across. That sounds huge, and it is, at least until we start comparing it to other galaxies. Our neighboring Andromeda galaxy, for example, is some 220,000 light-years wide. Another galaxy, IC 1101, spans as much as 4 million light-years.
Ok, fine, but what the heck is a light-year?
Glad you asked. It’s one of the most commonly used celestial yardsticks, the distance light travels in one year. Light zips along through interstellar space at 186,000 miles (300,000 kilometers) per second (more than 66 trips across the entire United States, in one second).
Multiply that by all the seconds in one year, and you get 5.8 trillion miles (9.5 trillion kilometers). Just for reference, Earth is about eight light minutes from the Sun. A trip at light speed to the very edge of our solar system – the farthest reaches of the Oort Cloud, a collection of dormant comets way, way out there – would take about 1.87 years. Keep going to Proxima Centauri, our nearest neighboring star, and plan on arriving in 4.25 years at light speed.
If you could travel at light speed. Which, unless you’re a photon (a particle of light), you can’t, and, by current physics, might never be possible.
How Many Solar Systems in The Milky Way?
So far, astronomers have found more than 500 solar systems and are discovering new ones every year. Given how many they have found in our own neighborhood of the Milky Way galaxy, scientists estimate that there may be tens of billions of solar systems in our galaxy, perhaps even as many as 100 billion.
Our solar system is just one specific planetary system—a star with planets orbiting around it. Our planetary system is the only one officially called “solar system,” but astronomers have discovered more than 3,200 other stars with planets orbiting them in our galaxy. That’s just how many we’ve found so far. There are likely to be many more planetary systems out there waiting to be discovered!
You may think that our solar system is unique, but there are over 5,000 solar systems that have already been discovered and analyzed in detail. Every year scientists learn about more and more solar systems that are either different, similar, or dissimilar to ours. They also discover new solar systems every year.
With this in mind, scientists have estimated that there could be tens of billions of solar systems in our Milky Way alone. Many believe it could be as high as 100 billion.
Be that as it may, not only planetary systems exist within our galaxy. There are also star systems, which are stars that revolve around each other.
How Many Planets are in our Galaxy?
NASA estimates that there are at least 100 billion planets in our Milky Way alone. Others estimated that the Milky Way could have anywhere from 100 to 200 billion planets.
Over 4,000 exoplanets have now been discovered, and more are being discovered every day. These planets are either part of a planetary system or they are rogue planets.
Rogue planets are harder to spot because they don’t orbit a star. Take our solar system for example; There are eight planets and at least five dwarf planets orbiting our star, the Sun, a single star.
There might as well be a ninth planet, or what some call Planet X, but we’re still searching for it. If just one star, our sun, could house up to eight or nine planets, then surely our Milky Way, which contains billions and billions of stars, should also have billions and billions of planets.
How Many Planets in the Milky Way Can Support Life?
Scientists have estimated that one in five stars like our sun is orbited by at least one Earth-like planet that could support life. Based on mapping of our Milky Way and through simulations, there are an estimated 40 billion planets that could support life in our Milky Way.
However, this is an average number as there could be many more. There are many different factors when you consider this. However, one aspect that might put our calculus in a different perspective is this: We only know how life adapts and evolves based on the species that live and lived on our planet.
There is no telling where life in space is limited to, and thus many planets that we consider inhospitable to us may actually be hospitable to other unknown species. This may sound like science fiction, but there is a truth in it that cannot be denied; we do not know how life can evolve and adapt; we do not know its limits, shapes and forms. What we do know is that life exists, adapts and evolves.