How Many Planets and Solar Systems are in our Galaxy?

Most of us are familiar with our own solar system. It has the Sun in its center with eight planets in orbit around. But did you know our Milky Way is home to millions of other solar systems and trillions of other planets?

Comparative sizes of planets discovered by Kepler with some of our planets for size comparison: image credit NASA/Kepler Mission/Wendy Stenzel

 

A solar (or planetary) system consists of at least one star and any objects that move in orbit around it. If you need a refresher on the differences between Solar Systems and Galaxies, please read our article: The Solar System versus a Galaxy versus the Milky Way versus the Universe.

Some Perspective on Distances
To find a new solar system you must first find a planet. Unfortunately planets outside our solar system are just too far away to be photographed by any telescope currently in existence. Here is a picture of Pluto taken by the Hubble Space Telescope. Although Pluto is almost the same size as our moon, the dwarf planet appears only as a small dot even in the eyes of our most powerful space telescope.

Pluto and its moon Charon as seen by Hubble-
Pluto and its moon Charon as seen by the Hubble Space Telescope in 2005. (credit: NASA, ESA, H. Weaver (JHU/APL), A. Stern (SwRI), and the Hubble Space Telescope Pluto Companion Search Team

To you and I, Pluto is very far away from Earth. The distance between our two planets can vary from 4.3 billion kilometers (when our orbits are close together) upwards to 7.5 billion kilometers (when our orbits take us to opposite sides of the Sun). Although that may seem like an incredible distance, when put in terms of Light Years instead of kilometers, Pluto is just a stone’s throw away from Earth.

A light year is the distance that a particle of light travels in one Earth year. Read why astronomers measure distance by time.

When we measure by the speed of light, we find that Pluto has a range of 4.3 to 7.5 light hours from Earth. That’s how long it take light (or any other form of electromagnetic radiation like radio waves) to travel from Earth to Pluto.

When NASA’s New Horizons spacecraft flew by Pluto in July 2015, the signals transmitted by the spacecraft to Earth were delayed by over four hours. That’s the time it took for the radio waves traveling at the speed of light to reach us from Pluto. When NASA was sending instructions to New Horizon, they had to wait more than eight hours to confirm that New Horizons received the signal (four hours to the spacecraft, another fours hours to receive the confirmation on Earth).

So if Hubble can barely image Pluto at a few light hours away, how do we handle the imaging of planets around stars that are so much further away? Did you know that the next closest star to the Sun is over 4.2 light years away! Sadly we don’t have a telescope large enough to reveal the individual planets around any stars outside of our Solar System.

If all goes to plan, the European Extremely Large Telescope will start scanning the skies by 2024. With 16 times more power than Hubble, this will be the first telescope that should be able capture mankind’s first images of another planet outside our Solar System.

 

We Can’t See Other Planets but we Can Measure Their Effects
Finding relatively tiny planetary bodies in the vast emptiness of space seems impossible; and it was impossible until just recently. It wasn’t until the 1990’s when science and technology advanced far enough to give us the tools needed to detect planets around other stars.

Arecibo Observatory
The 1000-foot radio collecting dish at the Arecibo Observatory in Arecibo, Puerto Rico. (credit: courtesy of the NAIC – Arecibo Observatory, a facility of the NSF)

The first exoplanet was detected in 1992 using the Arecibo Observatory, a giant Radio Telescope in Arecibo, Puerto Rico (more famously known as the giant radio telescope featured in the blockbuster movie Contact). While listening to a newly discovered pulsar over 2300 light years away (Pulsar PSR B1257+12 now named ‘Lich’), astronomer Aleksander Wolszczan heard irregular gaps in the steady beat of radio waves coming from the pulsar.

A pulsar is a tiny star that sends out short burst of radio waves as it rotates up to hundreds of times per second. An exoplanet or extrasolar planet is a planet that orbits a star other than the Sun.

He determined that these gaps were caused by orbiting planets around the pulsar. These planets were blocking the signals reaching his radio dish in regular repeating intervals. The world’s first exoplanets were eventually crowned PSR B1257+12 A, B and C but are now referred to by their common (and somewhat frightful) names Draugr, Poltergeist and Phobetor.

Unfortunately it turns out that planets around pulsars are extremely rare. But just a few years later in 1995, a team from the University of Geneva confirmed existence of another planet. Astronomers Michel Mayor and Didier Queloz discovered a planet that was approximately half the size of Jupiter. It was orbiting around a sun-like star only 50 light years away within the constellation Pegasus. This was the first planet detected by light instead of radio waves.

The new planet 51 Pegasi b (later named Dimidium) was detected by an incredibly sensitive instrument called a Spectroscope. By using a method of detection called Spectroscopic Radial Velocity, the instrument measures the “wobbliness” of its target star. When a planet orbits a star, the planet’s gravity tugs ever so slightly on the star. This gravitational force causes the star’s spin to slightly waver. These regular oscillations can then be detected by spectroscopes here on Earth. To date more than 700 planets have been detected using Spectroscopic Radial Velocity.

 

Dimming Turns Out to be a Bright Idea
There now exists a more effective way for detecting planets orbiting far-off stars. In 2009 the Kepler Space Observatory was launched with one main instrument on board called a Photometer. This instrument measures the magnitude that a star dims when a planet passes in front of it.

Between 2009 and 2013 Kepler pointed it’s photometer at a fixed field of approximately 145,000 stars, waiting to capture any changes in brightness that might over the next four years. Using this method (called Transit Photometry) Kepler has discovered over 1000 new exoplanets and has over 3000 other planetary candidates to confirm with future observation.

Kepler is also responsible for helping to discover KIC 8462852, the WTF Star in the constellation Cygnus that has astronomers looking for signs of alien civilizations.

Gravitational Microlensing of LRG 3-757
The luminous red galaxy LRG 3-757 in the constellation Leo bends light so strongly that it shows a distant blue galaxy 5 billion light years directly behind it. Together they are known as the Cosmic Horseshoe. (credit ESA/Hubble and NASA)

Unfortunately the Spectroscopic Radial Velocity method and the Transit Photometry method only work to find planets that orbit relatively close to their host stars. To detect planets that orbit much further from their star, a team at the Paris Institute of Astrophysics has recently developed a new detection method using gravitational microlensing.

Gravitational Microlensing is an observable result of Einstein’s General Theory of Relativity. When light passes by a massive object, the light appears to bends around the object. When light bends around an object in space, it will show a bright and magnified image of the area behind it.

The team’s lead author Arnaud Cassan explains that by measuring five years of changes in brightness due to gravitational microlensing in various parts of the sky, they were able to detect multiple light signatures of planets around stars, adding to our known tally of planets.

Since light has no mass and can not be affected by gravity, light itself does not actually bend. Instead gravity around a massive object in space causing time and space itself to distort. Light then follows the curved distortions of space-time caused by gravity, similar to light passing through a magnifying glass.

 

So How Many Planets are in our Galaxy?
If you add up all planetary discoveries using all of the above methods, you get an average of a whopping 160 planets per 100 stars. Since we think our galaxy contains between 200 to 400 billion stars, there should be around 320 to 800 billion planets within our Milky Way. If we then guess that most star systems contain an average of 5 to 10 planets, we safely say there are between 50 to 100 million other Solar Systems within our galaxy.

Furthermore we know that roughly 10 to 20 billion of these planets would orbit within the habitable zone of their star. These planets in just the right orbit are neither too close or too far away from their star to prevent life from forming on the planet. It stands to reason that billions and billions of Earth-like planets within our galaxy are waiting to be discovered.

If you thought that was a lot of planets, we haven’t even talked about the one that exist in-between the stars. Planets that are not gravitationally tied to a star are called Unbound or Rogue Planets. They freely zip through space, changing their courses whenever they interact with gravity from any other massive object. We think that there may be more unbound vs. bound planets in our galaxy. That would bring our galaxy’s planetary total well past two trillion planets!

Astronomers know that the true size and scale of even our own local galaxy is extremely hard for most to comprehend. If you suddenly feel small and insignificant, we can certainly appreciate why.

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