How to Look for Habitable Planets Thousands of Light-years Away?

by Carson

If you are interested in space, you likely have this question, “Are we alone?” To answer this, we have to look for habitable planets and moons, whether it’s right within our Solar System or thousands of light-years away. In this article, we will explain how to do so. (We’ll specifically talk about exoplanets in this article because they cannot be visited by spacecraft).

The Discovery of Exoplanets

Firstly, we have to look at how exoplanets are found in the first place before even exploring their features. The process is quite challenging, but there are already some proven ways to find these small, virtually invisible objects. Fortunately, some of the methods can be used to find out the physical features of the exoplanet, too!


The most successful way to find a planet orbiting a star is through transit. Imagine that you put something in front of a light source (don’t use the Sun as it can harm your eyes in the process)! The light will dim or become obscured entirely.

That’s precisely the thing that makes the transit possible to observe. Note that they may not see the shadows of the planets themselves, but it’s the modest increase in apparent magnitude (decrease in apparent brightness) that makes the planet’s discovery possible.

In a transit, starlight is dimmed due to the object that is in front of it
Image Credit: NASA/JPL Eyes, Canva
Radial Velocity

Although the fact that the planets orbit the Sun is quite obvious, it’s not 100% true. The planets, asteroids, comets, and the Sun itself orbits the barycenter of the Solar System. The Sun’s movements are relatively insignificant because the Sun is much more massive than the planets, which also exerts a gravitational force to our star. However, the barycenter of Jupiter and Sun lies just outside the Sun’s surface!

This applies to exoplanets and is a useful method of detecting exoplanets. The important helper of that method is the Doppler effect. That lowers the wavelength if the object moves closer to us and extends the wavelength if the object moves further from us. The Doppler effect of a wobbling star is very insignificant, but it’s measurable.

A planet and a star orbiting their barycenter
Image Credit: NASA/JPL Eyes, Canva
Direct Imaging and Gravitational Lensing

Remarkably, even though exoplanets are very dim and are often obscured by its star, we can take photos of it directly. This is possible by removing the glare of the star and resolving objects around it.

As a rule of thumb, gravity bends light. If The apparent brightness of the star increases and decreases, it might point to a planet that is bending the light of the star. However small and insignificant the planets are, their effects on starlight are still detectable. This method is even used to find rogue planets that don’t orbit a star!

Finding Out the Features

Now, it’s probably the most fantastic part: Tracking down important features of a planet from hundreds, if not thousands of light-years away. In reality, its size, mass, orbit, and atmospheric composition (and more) can be determined on its discovery if the star’s size and mass are found.

Size, Mass, and Density

Firstly, we can use the reduced brightness of the star to figure out the size of the planet. The more the star dims, the larger the planet. Then, we can utilize the radial velocity method to determine its mass. The more the star wobbles, the more massive the planet. The data of the size and mass combined can provide the density of the planet, which can indicate if it’s a terrestrial planet or a gas giant. Rocky planets tend to be small and have a higher density, while gas giants are usually large and have a lower density.


By observing how fast the transit goes, scientists can figure out the orbital period of the planet. With the mass of the star, we can calculate how far the planet is from its star. The distance of the planet from the star is essential because it decides whether the planet falls into the habitable zone, which is important if we have to find life as we know it. If the object lies outside the habitable zone, it will either be too hot or too cold for water to stay liquid.

An illustration of the habitable zone
Image Credit: NASA

Note: Life may exist outside the habitable zone because liquid water may lie inside the planet’s surface, or alien life may be adapted to survive in other conditions.

Atmospheric Composition

Another critical factor is the atmospheric composition. If the gases are toxic or the atmospheric pressure is not right, life as we know it cannot survive. During a transit, we can find the atmospheric composition by inspecting the spectrum of the atmosphere of the planet. If something is missing and the spectrum of a particular element or compound matches, that means this type of chemical is present.


In this article, we discussed how to look for habitable planets using transit, radial velocity, and the star’s size and mass. It is no easy feat as it requires sensitive equipment and careful data analysis, but it’s worth the effort. If you are curious and want to learn more, please visit the articles in the references below. Moreover, if you found other ways of finding clues of a habitable planet, please leave it in the comments below.

References and Credits

  1. Exoplanet Exploration: Planets Beyond our Solar System. (n.d.). 5 Ways to Find a Planet. Retrieved May 4, 2021, from
  2. NASA Space Place. (2020, June 3). What Is a Barycenter? Retrieved May 4, 2021, from
  3. CrashCourse. (2015, August 6). Exoplanets: Crash Course Astronomy #27. Retrieved May 4, 2021, from
  4. The Planetary Society. (n.d.). What Is the Habitable Zone? Retrieved May 4, 2021, from
  5. Exoplanet Exploration: Planets Beyond our Solar System. (2021, April 28). The Searchers: How Will NASA Look for Signs of Life Beyond Earth? Retrieved May 4, 2021, from
  6. Exoplanet Exploration: Planets Beyond our Solar System. (2021, April 2). The Search For Life. Retrieved May 4, 2021, from
  7. (n.d.). Exo Pioneer. Retrieved May 4, 2021, from

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