How many planets are there in the Universe?

For most of history, our Solar System contained the only known planets.

Although we now believe we understand how the Sun and our Solar System formed, this early view is an illustration only. When it comes to what we see today, all we have left are the survivors. What was around in the early stages was far more plentiful than what survives today, a fact that is likely true for every successful stellar system and also every failed star system in the Universe. (Credit: JHUAPL/SwRI)

Yet each twinkling point of light — each star — represents a new chance.

Each star within each galaxy contains its own stellar system, and potentially, its own set of planets. For a long time, we didn’t know how many of these stars actually possessed planets or what the likelihood of planets of different masses were. Today, more than 30 years after the first exoplanet was discovered, we are closer than ever to understanding just how many planets populate our Universe. (Credit: ESO/VISTA/J. Borissova)

Such systems could contain rocky planets, possessing oceans, continents, and — perhaps — life.

The surfaces of six different worlds in our Solar System, from an asteroid to the Moon to Venus, Mars, Titan, and Earth, showcase a wide diversity of properties and histories. While Earth is the only world known to host life, these other worlds may someday expand our current understanding of how frequently life arises. (Credit: Mike Malaska; ISAS/JAXA, NASA, IKI, NASA/JPL, ESA/NASA/JPL)

In the early 1990s, the first planetary detections around other stars arrived.

If we want to know how many planets there are in the Universe, one way to make such an estimate is to detect planets to the limits of an observatory’s capabilities, and then to extrapolate how many planets there would be if we viewed it with a limitless observatory. Although there remain tremendous uncertainties, we can now safely say that the average number of planets-per-star is greater than 1. (Credit: ESO/M. Kornmesser)

As planets orbit their stars, those stars orbit their mutual center-of-mass, creating “wobbles” in their motion.

When a massive planet orbits its parent star, the star and planet will both orbit their mutual center of mass. Even if the planet is not directly observable, its presence, orbital period, and mass (multiplied by an uncertain angle-of-orbital-inclination) can be extracted simply by measuring the periodic motion of the parent star with the method of Doppler spectroscopy. (Credit: European Southern Observatory)

This stellar wobble, or radial velocity, reveals planetary masses and orbital periods, up to an uncertain inclination angle.

Today, exoplanets that cannot be directly seen or imaged can still be detected through their gravitational influence on their parent star, which causes a periodic spectral shift that can be clearly observed. (Credit: E. Pécontal)

Meanwhile, transiting planets obscure a portion of their parent star’s light.

When planets pass in front of their parent star, they block a portion of the star’s light: a transit event. By measuring the magnitude and periodicity of transits, we can infer the orbital parameters and physical sizes of exoplanets. When transit timing varies and is followed (or preceded) by a smaller-magnitude transit, it may indicate an exomoon as well, such as in the system Kepler-1625. (Credit: NASA’s GSFC/SVS/Katrina Jackson)

This periodic dimming reveals a planet’s radius and period; it’s responsible for most presently discovered planets.

The candidate rogue planet CFBDSIR2149, as imaged in the infrared, is a gas giant world that emits infrared light but has no star or other gravitational mass that it orbits. It is one of the only rogue planets known, and was only discoverable because its large-enough mass emits its own infrared radiation. (Credit: ESO/P. Delorme)

Meanwhile, direct imaging and microlensing also reveal exoplanets; their numbers may skyrocket in coming decades.

When a gravitational microlensing event occurs, the background light from a star gets distorted and magnified as an intervening mass travels across or near the line-of-sight to the star. The effect of the intervening gravity bends the space between the light and our eyes, creating a specific signal that reveals the mass and speed of the planet in question. (Credit: Jan Skowron/Astronomical Observatory, University of Warsaw)

With 400 billion Milky Way stars, we estimate they contain 1-to-10 trillion orbiting planets, total.

Although the Milky Way is full of stars, this stellar density map of the sky, constructed with data from the ESA’s space-based Gaia mission, is only accurate to the extent that visible light gives us accurate information. The ultraviolet and visible light emitted by the Milky Way’s stars is obscured by the light-blocking dust in our galaxy, requiring longer-wavelength views to reveal them. Through a combination of multiwavelength observations and inferences about the low-mass stars in our galaxy, we now estimate that there are ~400 billion stars within the Milky Way, and 80% of those are M-class red dwarfs. (Credit: ESA/Gaia)

Meanwhile, rogue/orphan planets — ejected and/or formed without parent stars — could be 10-to-10,000 times as numerous.

Rogue planets may have a variety of exotic origins, such as arising from shredded stars or other material, or from ejected planets from solar systems, but the majority should arise from star-forming nebula, as simply gravitational clumps that never made it to star-sized objects. When a microlensing event occurs, we can use the light to reconstruct the intervening planet’s mass. (Credit: C. Pulliam, D. Aguilar/CfA)

With ~2 trillion galaxies within our observable Universe, we can extrapolate our Universe’s planetary total.

The Hubble eXtreme Deep Field (XDF) may have observed a region of sky just 1/32,000,000th of the total, but was able to uncover a whopping 5,500 galaxies within it: an estimated 10% of the total number of galaxies actually contained in this pencil-beam-style slice. The remaining 90% of galaxies are either too faint, or too red, or too obscured for Hubble to reveal, but when we extrapolate over the entire observable Universe, we expect to obtain a total of ~2 trillion galaxies within the visible Universe. (Credit: HUDF09 and HUDF12 teams; Processing: E. Siegel)

There are ~10²⁵ planets that orbit stars, with some ~10²⁶-10³⁰ additional starless planets.

When starlight passes through a transiting exoplanet’s atmosphere, signatures are imprinted. Depending on the wavelength and intensity of both emission and absorption features, the presence or absence of various atomic and molecular species within an exoplanet’s atmosphere can be revealed through the technique of transit spectroscopy. (Credit: ESA/David Sing/PLAnetary Transits and Oscillations of stars (PLATO) mission)

With a little luck, we’ll soon find the first extra-solar planet housing extraterrestrial life.

The Drake equation is one way to arrive at an estimate of the number of spacefaring, technologically advanced civilizations in the galaxy or Universe today. However, it relies on a number of assumptions that are not necessarily very good, and contains many unknowns that we lack the necessary information to provide meaningful estimates for. (Credit: University of Rochester)

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.