How To Find a Planet

Have you ever wondered how many other Earths can there be in the vast universe? Given the idea that in our Milky Way alone there are 100 billion stars, one would think that there must be many planets out there like our Earth. Well, the short answer to that question is, scientists are still looking. Thousands of clusters of planers have been found orbiting a star by the spacecraft Kepler, yet identifying them as habitable it’s a tricky┬áprocess. They are so far away that in a photograph by a┬áspace observatory, like Kepler, they would merely be seen as two or three pixels in length. What Kepler does is categorize the planets it finds as candidates for scientists to analyze later. Let’s break down all the information we know.

Artist's concept depicts Kepler-186f

The artist’s concept depicts Kepler-186f , the first validated Earth-size planet to orbit a distant star in the habitable zone Credits: NASA Ames/SETI Institute/JPL-Caltech

To know what to look for, scientists looked at our solar system and most importantly our Earth’s history. This gives clues to scientists on what to look for on another planet that could harbor life.

  1. The Goldilocks Zone
    Also known as, the Habitable zone, this area marks the circumference around a star where it’s not too close and not too far. If a planet is too close, it would be too hot and water would boil away. If a planet is far away from its start, it would be too cold and all the water would freeze.

    Credits: NASA’s Goddard Space Flight Center

    2) Planet Size
    The size of the planet greatly influences its ability to have an atmosphere that is suitable for living organisms. If a planet is too small it’s atmosphere may not be thick enough to retain oxygen. An example of this would be our moon. Despite being within the Habitable zone, its small size prevents it from keeping a fitting atmosphere to sustain living organisms.
    On the other hand, if the planet is too big then gases in the atmosphere create too much pressure on the surface of the planet.

Kepler Spacecraft

The Kepler spacecraft, named after the 17th-century German mathematician, astronomer, and astrologer Johannes Kepler, was launched into space on March 6, 2009, with the mission to look for Earth-size planets in our galaxy. Kepler continuously looks at a specific region in the Cygnus and Lyra constellations of our Galaxy, continuously monitoring all the stars.

Kepler's six years of findings up to up to May 2015

The Transit Method

Kepler uses the Transit Method to look for planets around a star. The transit method measures the amount of light we see from a star and how it varies when a planet passes in front of it, this is called “transit”. When a planet moves in front of its star the brightness dips. Kepler measures this dips in brightness to determine the planet’s orbit. This measurement helps determine the planet’s radius.
The loos of some of Kepler’s instruments led to new missions, learn about them here.



The Radial Velocity Method

Another way of finding exoplanets is by using the Radial method, also known as, the Doppler Wobble Method. This method measures how much a star wobbles. This wobble movement relates to the mass of a planet as the star tugs on it during its orbital cycle.
Scientists measure this wobbles by the change of color the light from the star emits. As a start moves away from us, light waves leaving the star are stretched and red. As the star moves toward us the light waves are compressed and blue. Though this method you’re able to measure the mass of the planet.

Radial Velocity Method chart


Having determined the planets’ radius from the Transit Method and the planet’s mass from the Radial Velocity Method, it is possible to calculate the planets’ average density. the denser a planet is, the more likely that planet is rocky; and if a planet is less dense them the planet is more likely glass.

planet density, our solar system

So far these are the closest candidates Earth-size planets:

  1. NASA’s Kepler Discovers First Earth-Size Planet In The ‘Habitable Zone’ of Another Star
  2. ESO Discovers Earth-Size Planet in Habitable Zone of Nearest Star


Resources for Educators

  1. What is Habitable Zone video
  2. NASA: Solar System and Beyond