On a clear night, and also when light air pollution isn’t a serious factor, looking up at the skies is a breathtaking experience. ~ above occasions favor these, the is simple to it is in blown far by the sheer variety of stars out there. But of course, what we have the right to see on any type of given night is just a portion of the variety of stars that in reality exist in ~ our Galaxy.

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What is even much more astounding is the concept that the bulk of these stars have actually their own system of planets. For part time, astronomers have believed this to be the case, and ongoing research appears to confirm it. And this normally raises the question, just how countless planets space out there? In our galaxy alone, surely, there must be billions!

Number that Planets every Star:

To important answer that question, we need to crunch part numbers and account for part assumptions. First, despite the discovery of hundreds of extra-solar planets, the Solar mechanism is quiet the only one the we have studied deeply. Therefore it might be that ours possesses much more star solution than others, or that our Sun has actually a portion of the planets that various other stars do.


So let’s assume that the eight planets the exist within our Solar system (not taking into account Dwarf Planets, Centaurs, KBOs and other bigger bodies) stand for an average. The next step will certainly be to multiply that number by the amount of stars that exist within the Milky Way.

Number of Stars:

To it is in clear, the actual variety of stars in the Milky means is subject to part dispute. Essentially, astronomers are compelled to do estimates due to the truth that us cannot view the Milky means from the outside. And given the the Milky method is in the form of a barred, spiral disc, the is difficult for united state to watch from one next to the other – many thanks to irradiate interference indigenous its many stars.

As a result, approximates of how plenty of stars there space come down to calculations of our galaxy’s mass, and also estimates of how much of that mass is comprised of stars. Based upon these calculations, researchers estimate the the Milky means contains between 100 and 400 exchange rate stars (though some think there might be as numerous as a trillion).


Doing the math, we have the right to then say the the Milky means galaxy has – on average – in between 800 billion and 3.2 sunshine planets, with some approximates placing the number as high a 8 trillion! However, in order to determine just how many of them room habitable, we need to think about the number of exoplanets found so much for the benefits of a sample analysis.

Habitable Exoplanets:

As that October 13th, 2016, astronomers have confirmed the presence of 3,397 exoplanets native a list of 4,696 potential candidates (which to be discovered in between 2009 and 2015). Several of these planets have been it was observed directly, in a procedure known as straight imaging. However, the vast majority have been detected indirectly utilizing the radial velocity or transit method.

In the case of the former, the presence of planets is inferred based on the gravitational influence they have on their parental star. Essentially, astronomers measure how much the star moves ago and soon to identify if it has actually a mechanism of planets and also how huge they are. In the instance of the transit method, planets room detected once they pass directly in former of your star, resulting in it come dim. Here, size and mass room estimated based on the level the dimming.


In the food of that is mission, the Kepler mission has actually observed around 150,000 stars, which during its initial 4 year mission consisted mostly of M-class stars. Also known as red dwarfs, these low-mass, lower-luminosity stars space harder to observe than our very own Sun.

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Histogram reflecting the variety of exoplanets discovered by year. Credit: NASA Ames/W. Stenzel, Princeton/T. Morton

Since the time, Kepler has gone into a new phase, likewise known together the K2 mission. Throughout this phase, which started in November that 2013, Kepler has actually been moving its focus to observe an ext in the means of K- and G-class stars – i beg your pardon are practically as bright and hot together our Sun.

According come a recent study native NASA Ames research Center, Kepler discovered that about 24% that M-class stars may harbor potentially habitable, Earth-size planets (i.e. Those the are smaller than 1.6 time the radius that Earth’s). Based ~ above the variety of M-class stars in the galaxy, that alone represents around 10 billion possibly habitable, Earth-like worlds.


Meanwhile, analyses that the K2 phase says that about one-quarter of the bigger stars surveyed may likewise have Earth-size planet orbiting within your habitable zones. Take away together, the stars it was observed by Kepler do up around 70% of those found within the Milky Way. So one have the right to estimate that there space literally tens of billions of potentially habitable planets in our galaxy alone.

In the comes years, brand-new missions will certainly be launching, favor the James Webb an are Telescope (JWST) and also the Transitting Exoplanet survey Satellite (TESS). These objectives will have the ability to detect smaller planets orbiting fainter stars, and maybe also determine if there’s life on any type of of them.

Once these brand-new missions acquire going, we’ll have better estimates that the size and variety of planets the orbit a typical star, and we’ll be able to come increase with better estimates that just numerous planets there room in the galaxy. However until then, the numbers are still encouraging, together they indicate that the opportunities for extra-terrestrial intelligence are high!


We have actually written countless articles around galaxies for cosmos Today. Here’s How countless Stars are there in the Milky Way?, How many Planets are there in the Solar System?, What space Extra-Solar Planets?, Planets Plentiful around Abundant Red Dwarf Stars, examine Says, Life after ~ Kepler: Upcoming Exoplanet Missions.

If you’d like much more info ~ above galaxies, check out Hubblesite’s News publication on Galaxies, and here’s NASA’s Science page on Galaxies.

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We have also recorded an illustration of Astronomy Cast around galaxies – episode 97: Galaxies.