Kepler Planets Lets Us See What Lies Beyond Our Solar System

The Kepler space telescope spent nine years in deep space hunting for exoplanets and finding there were far more than scientists thought. NASA/Ames Research Center/W. Stenzel/D. Rutter

One of the biggest questions humans have been pondering for decades, if not longer, is whether there is life on other planets. We still don't have a definitive answer to that question yet. But thanks to NASA's Kepler space telescope, we're closer to finding one.

Named after famous German astronomer Johannes Kepler, Kepler's mission was to find extrasolar planets, particularly those that might be in or near habitable zones. An extrasolar planet (aka an exoplanet) is any planet orbiting a star that's beyond our solar system. A star's habitable zone is the distance from that star where liquid water could exist on an orbiting planet's surface, thus indicating the possibility for life. It's usually an area that's not too hot and not too cold — conditions that would be right for liquid water (as opposed to water vapor).

The Kepler space telescope launched in 2009 and was managed by NASA's Jet Propulsion Laboratory. Its mission was to observe more than 150,000 planets in one part of the night sky simultaneously. So, what did we learn from these Kepler planets?

What Kepler Found

This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. NASA/JPL-Caltech/Ames

The mission was a resounding success. More than 2,600 exoplanets were discovered during its operational phase from 2009 to 2018, with thousands of additional planets uncovered in subsequent years using Kepler data. Here are some of the key findings from the Kepler mission.

There are actually many more planets than stars —perhaps 10 planets for each star— and these planets are quite diverse.
The most common type of planet (a planet between the size of Earth and Neptune) is one that doesn't even exist in our solar system.
Small (i.e., Earth-sized), rocky planets are more common than gassy giants like Saturn and Jupiter.
Solar systems are diverse too. Our inner solar system has four planets orbiting its star (the sun) but other systems have as many as eight planets orbiting its star.

"We now absolutely know for sure that other planets exist," says Dr. Steve Howell, a senior NASA research scientist who worked on the Kepler mission. "Many, many other planets. Probably no one doubted that, but there was no proof until Kepler."

How Kepler Found Planets

Astronomers discovered the first two exoplanets relatively recently in 1992. Today we know of more than 5,400 others, with more than 9,600 candidates awaiting confirmation. This is largely thanks to the Kepler mission and its then-novel transit method of discovery.

During the mission, the Kepler spacecraft used a special telescope and image sensor array to measure the light coming from a given star over time. If the star's light repeatedly dimmed at regular intervals, this indicated a planet was orbiting the star.

Studying transits can also help scientists determine exoplanet characteristics. For example, scientists can calculate the size of an exoplanet's orbit based on how long it takes to go around the star, as well as the planet's overall size, based on how much the star's light dims. Planets discovered using the transit method were called Kepler planets.

Scientists could even determine an exoplanet's atmospheric composition as it transits by analyzing the light that passed through its atmosphere and how it was dispersed. A planet's atmosphere is an important element in determining habitability.

Astrophysics and Space Research

Scientists believe that if there are exoplanets supporting life as we know it, they will resemble the Earth. This means they'll be smaller, as smaller planets tend to have a lighter, breathable atmosphere, and feature rocky terrain, important for offering trees, plants and land for living. They will also need to be in their star's habitable zone, ensuring the liquid water that's critical to life.

Perhaps the most exciting Kepler discoveries were the Earth-like planets that could possibly support life as we know it. Pondering whether there is other life in the universe is simply a human condition, Howell says.

In 15th-century Europe, he notes, people wanted to cross the ocean to see what was there, despite the voyage being as challenging as space exploration is today. And, of course, when they crossed the ocean, they found other human beings. "So, I think it's just become a very pressing question now, more so than philosophical," says Howell. "Are we alone? Now we have the means to try and get an answer."

In 2013, two of the Kepler spacecraft's reaction wheels failed so the mission was redesigned. The K2 mission involved turning the telescope about four times a year to different fields of view, as opposed to observing one patch of sky. Kepler still found exoplanets, but at a slower pace along with exploded stars and supermassive black holes.

Notable Exoplanets

The system Kepler-444, as shown in this artist's conception, formed when the Milky Way galaxy was just 2 billion years old. The tightly packed system is home to five planets; the smallest one is around the size of Mercury, and the largest is the size of Venus. Tiago Campante/Peter Devine

The exoplanets discovered during the Kepler mission, or later using Kepler data, are quite diverse and include hot, cold, rocky and gaseous planets. The mission even uncovered a "puffy" planet featuring a density akin to styrofoam. "We don't even understand how that could be possible," Howell says.

Here are a few of the more notable exoplanets the Kepler mission discovered, including some of Howell's favorites.

Kepler-13b

Roughly the size of Jupiter, exoplanet Kepler-13b orbits a star (Kepler-13a) that's part of a binary star system, with the second star being Kepler-13b. "Imagine if you could live on that planet," Howell says. "You would have this other sun in your night sky, basically, so you wouldn't have much of a night sky."

Kepler-22b

This was the first planet discovered in the habitable zone of a star outside of our solar system. "That planet is a little larger than Earth and is thought to be entirely covered with very, very deep oceans," Howell says.

Kepler-186f

Its claim to fame is that it was the first Earth-sized planet found within its star's habitable zone. Before this, all planets found in the habitable zone were at least 40 percent larger than Earth. Kepler-186f orbits its star every 130 days and gets a third the energy from its star that Earth gets from our sun, meaning that at high noon, its star appears as bright as our sun looks an hour before sunset.

Kepler-444

An ancient system, Kepler-444 contains five planets set fairly close to one another. Scientists believe it was formed about the same time as the Milky Way galaxy, or roughly 2 billion years ago. The five planets orbit their star in less than 10 days.

Kepler-10c

When this planet was discovered in 2014, it was originally hailed as a "mega-Earth" because of its size and the belief that it was solid. Although it seemed to defy conventional wisdom, scientists later found that this Earth-like planet wasn't so "Earth-like" after all. Later analysis determined Kepler-10c was about seven Earth masses, rather than the 17 previously thought. Also, it was not a rocky planet, but rather a typical volatile-rich planet, composed mainly of water.

K2-417b

This was one of the last planets Kepler found, part of a group of three planets discovered thanks to a team of amateur and professional astronomers. K2-417b (sometimes written as K2 417 b) is a little more than three times the size of Earth and orbits a red dwarf star every 6.5 days. A paper about the three planets was published in May 2023 in the Monthly Notices of the Royal Astronomical Society.

Future Discoveries

In 2018, the Kepler telescope ran out of fuel and its exoplanet-hunting role was taken over by Transiting Exoplanet Survey Satellite (TESS). Also on hand is the long-awaited James Webb Space Telescope. The biggest and most powerful telescope ever built, one of its objectives is to look for signs of life on other planets. Howell, for one, is sure there is.

"The chances that the Earth is the only place in the entire universe that happened to evolve some sort of life, I think that that can't be true," he says. "I would encourage everybody to observe and look up and find out what you can, because there's a lot of stuff we don't know yet."