# Aliens

Published scientific estimates of habitable worlds and alien civilizations, each with its source.

## Milky Way
- **100–400 billion**: stars in our galaxy, which is about 100,000 light-years across. Source: [NASA Science](https://science.nasa.gov/universe/exoplanets/our-milky-way-galaxy-how-big-is-space/)
- **300 million**: potentially habitable rocky planets around Sun-like stars, a conservative count. The nearest is likely about 20 light-years away. Source: [Bryson et al. 2021, The Astronomical Journal; NASA](https://science.nasa.gov/universe/exoplanets/about-half-of-sun-like-stars-could-host-rocky-potentially-habitable-planets/)
- **~1 billion**: Earth-like planets in the galaxy today. Source: [Behroozi & Peeples 2015, Monthly Notices of the Royal Astronomical Society](https://arxiv.org/abs/1508.01202)
- **11%**: of Sun-like stars host an Earth-size planet receiving about as much light as Earth does (1–4 times). Source: [Petigura, Howard & Marcy 2013, PNAS](https://doi.org/10.1073/pnas.1319909110)
- **At least 36**: communicating civilizations right now (range 4–211), the nearest within about 17,000 light-years. Assumes intelligence takes about 5 billion years to arise, as it did here, and civilizations broadcast for 100 years. Under looser assumptions: at least 928. Source: [Westby & Conselice 2020, The Astrophysical Journal](https://arxiv.org/abs/2004.03968)
- **1 in 60 billion**: If the odds that a habitable planet ever produces a technological species are better than this, we are probably not the first in the galaxy. Source: [Frank & Sullivan 2016, Astrobiology; NASA](https://science.nasa.gov/universe/exoplanets/are-we-alone-in-the-universe-revisiting-the-drake-equation/)

## Andromeda
- **~1 trillion**: stars, about 2.5 million light-years away. Source: [ESA/Hubble 2025](https://esahubble.org/news/heic2501)
- **11% and 6.5%**: of all stars ever formed in Andromeda may have planets with basic and complex life. The richest zone is a ring 12–14 kiloparsecs from the centre. Source: [Carigi, García-Rojas & Meneses-Goytia 2013, Revista Mexicana de Astronomía y Astrofísica](https://arxiv.org/abs/1208.4198)
- **16 vs 8**: kiloparsecs from the centre: where the most stars with habitable planets sit in Andromeda versus the Milky Way. In Andromeda that peak sits twice as far out. Source: [Spitoni, Matteucci & Sozzetti 2014, Monthly Notices of the Royal Astronomical Society](https://doi.org/10.1093/mnras/stu484)

## Observable universe
- **~10²⁰**: Earth-like planets (100 billion billion). Earth formed after 80% of them. Source: [Behroozi & Peeples 2015, Monthly Notices of the Royal Astronomical Society](https://arxiv.org/abs/1508.01202)
- **92%**: at least, the chance we are not the only civilization the universe will ever have. More than ten times as many planets are still to form as exist today. Source: [Behroozi & Peeples 2015, Monthly Notices of the Royal Astronomical Society](https://arxiv.org/abs/1508.01202)
- **1 in 10²⁴**: If the odds per habitable planet are better than this, humanity is not the only time technological intelligence has evolved. Source: [Frank & Sullivan 2016, Astrobiology](https://arxiv.org/abs/1510.08837)

## The Drake equation

N = R* × fp × ne × fl × fi × fc × L

Frank Drake's 1961 ranges (as given in Drake & Sobel 1992, Is Anyone Out There?):
- R* (New stars formed per year in the galaxy): 1
- fp (Fraction of stars with planets): 0.2 to 0.5
- ne (Habitable planets per planetary system): 1 to 5
- fl (Fraction of those where life appears): 1
- fi (Fraction of those where intelligence evolves): 1
- fc (Fraction of those that become detectable): 0.1 to 0.2
- L (Years a civilization stays detectable): 1,000 to 100,000,000

Low values give N = 20; high values give N = 50,000,000.