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An artist’s impression of a habitable exoplanet orbiting a red dwarf star

An artist’s impression of a habitable exoplanet orbiting a red dwarf star  (Ph03nix1986, Wikimedia Commons)

STEM in Context

Searching for exoplanets around the star next door

Emily Pass

Summary

In planetary science, astronomers have found planets that orbit other stars, just like Earth orbits the Sun. Some of them might even be able to support life.

How many planets can you name? Seven? Eight? You don’t have to stop after Uranus and Neptune. Scientists estimate that there could be billions of planets that orbit other stars, just like Earth orbits the Sun.

Scientists call these distant planets extrasolar planets, or exoplanets for short. “Extrasolar” just means ”outside the solar system.” Humans have observed the stars for thousands of years, but the first exoplanet wasn’t confirmed until 1991. Since then, scientists have discovered thousands more.

Exoplanets Explained (2013) by the staff at the Exolab at Caltech (2:20 min.).

Finding “invisible” exoplanets

 

Our solar system’s nearest neighbour is a star called Proxima Centauri. So far, astronomers have discovered one exoplanet orbiting this star. If you’ve ever seen Venus—the nearest planet to Earth—in the early evening or early morning sky, you would know that it is recognizably bright. But depending on where Venus is in its orbit, Proxima Centauri is a whopping 200 000 to 1 000 000 times farther away from Earth than Venus is!

If Venus looks like a star when you observe it from Earth, then moving it 1 000 000 times further away would make it invisible to the naked eye. So how do astronomers manage to find exoplanets orbiting around Proxima Centauri and beyond?

One important tool scientists use is called the transit method. Think of a solar eclipse. The moon passes between the Earth and the Sun, blocking part of the Sun’s light. In the same way, exoplanets sometimes pass between their host stars and Earth, blocking part of the star’s light. This causes the star’s brightness to dim slightly when viewed from Earth. Human eyes can’t detect these changes, but modern telescopes and computers can.

Graph and diagram showing how the brightness of a star changes as a planet passes in front of it
Graph and diagram showing how the brightness of a star changes as a planet passes in front of it (Source: Transiting Exoplanet Survey Satellite (TESS), NASA).

Did you know?

Some exoplanets are called rogue planets. These bodies seem to wander the cosmos, not orbiting any star at all.

Liquid water means a planet could be “just right” for life

Most exoplanets have been found using NASA’s Kepler Space Telescope. It uses the transit method to search our galaxy, the Milky Way. A new telescope called TESS (the Transiting Exoplanet Survey Satellite) was also launched in 2018. TESS uses the same transit method to search for Earth-like planets that are located around stars nearby to Earth.

Did you know?

Using data from the Kepler Space Telescope, scientists have estimated that there are tens of billions of Earth-sized planets in their star’s habitable zone. And that’s just in the Milky Way galaxy!

Why is it called the Goldilocks zone? If a planet is too close to its star, it would be too hot. Any water on the surface would evaporate, and the planet would suffer from a runaway greenhouse effect. If a planet is too far from its star, it would be too cold. Any surface water would be frozen solid. But if the distance between a planet and its star is just right, water could be liquid. Just like on Earth, which is in the Sun’s Goldilocks zone!

Liquid water is essential for life as we know it to exist. That’s why scientists pay so much attention to the Goldilocks zone. However, being in the Goldilocks zone doesn’t guarantee that a planet can host life.

Goldilocks zone
Some of the planets in the Goldilocks zone (Chester Harman [CC BY-SA 4.0] via Wikimedia Commons).

For example, solar flares could make a planet in the Goldilocks zone uninhabitable. Solar flares are violent bursts of energy that come from a star. If the exoplanet’s host star has strong solar flares, they could tear the planet’s atmosphere away and cause any liquid water to be lost to space. In fact, NASA’s MAVEN mission in 2014 has led scientists to believe this is what happened to Mars approximately four billion years ago, back when the Sun’s storms were far more violent.

Did you know?

As of January 2020, astronomers have confirmed the existence of 4 116 exoplanets.

Meet our neighbour, Proxima b

In the summer of 2016, astronomers announced big news about the planetary system next door. Proxima Centauri has a planet in its habitable zone!

The planet is called Proxima b. However, it’s not necessarily a second Earth. Like many Goldilocks zone exoplanets, Proxima b orbits a red dwarf star. Red dwarf stars like Proxima Centauri are smaller and colder than the Sun. This means their habitable zones are very close to their stars.

Artist's impression of the exoplanet Proxima b
Artist's impression of the exoplanet Proxima b (Source : ESO/M. Kornmesser [CC BY 4.0] via Wikimedia Commons).

Because Proxima b is in a tight orbit with Proxima Centauri, a year on the planet is really short: only 11 days! This closeness suggests that Proxima b is tidally locked to Proxima Centauri. Tidal locking occurs when a planet or moon’s orbital period is the same as its rotational period. For instance, the Moon is tidally locked to Earth, so the same side of the Moon always faces us on Earth. If Proxima b is truly tidally locked, that means one side of the planet permanently faces the scorching star, while the other side freezes in eternal darkness. And since it’s so near its star, Proxima b is also at risk of those atmosphere-destroying solar flares.

But the planet may have its own Goldilocks zone. There could be a band of perpetual twilight between the hot and cold faces, where the temperature is just right for life. And if life can exist on Proxima b, it will have a much brighter future than life on Earth. The Sun will die in around five billion years, but a red dwarf like Proxima Centauri still has trillions of years left to burn!

In 2019, astronomers discovered that Proxima b might have a big sister! They detected Proxima c, a candidate planet orbiting a lot further from the star. Proxima c would be too cold for life. However, it may be an ideal planet for direct imaging. This is a method in which telescopes take photos of the planet itself, instead of studying it indirectly through methods like transits

Did you know?

2016 was a big year for exoplanet research. Discoveries announced in that year nearly doubled the number of known exoplanets.

Breakthrough Starshot: Setting a course for Proxima b

One day, Earth might no longer be habitable. If this happened, could your descendants blast off to Proxima b? Scientists would have to learn more about this planet first.

You might also think sending anything that far would be impossible, but some scientists would disagree. One example is the late scientist Stephen Hawking. In 2016, Hawking and entrepreneur Yuri Milner announced Breakthrough Starshot, a project to develop and send tiny space probes to Proxima Centauri within the next 20 years. Each of these little probes would only be the size of a postage stamp. They would be accelerated to a fifth of the speed of light using huge lasers, reaching Proxima Centauri after a 20-year journey. This is an ambitious goal. To travel at a fifth of the speed of light, the probes will be moving away from the solar system 600 times faster than any spaceship ever built!

Is Breakthrough Starshot possible? Maybe. There are huge technical obstacles to overcome, inventions to invent, problems to solve. But once the technology exists, Proxima Centauri wouldn’t be the limit. Scientists could explore the hidden corners of the solar system and countless other planetary systems.

Will Starshot find life out there? Is there life out there? In 40 years, humanity may know. And by then, you might even be the scientist discovering it.

 

Starting Points

Connecting and Relating
  • Would you like to know if it is possible to sustain life on other planets? Why or why not?
  • What would you be interested in knowing about exoplanets?
  • Would you want to visit or live on another planet? 
Connecting and Relating
  • Would you like to know if it is possible to sustain life on other planets? Why or why not?
  • What would you be interested in knowing about exoplanets?
  • Would you want to visit or live on another planet? 
Relating Science and Technology to Society and the Environment
  • What different types of resources are needed to realize projects like Breakthrough Starshot?
  • Why does the discovery of planets outside of our solar system matter to society?
  • How does the discovery of so many potentially habitable planets outside our solar system affect beliefs about intelligent life in the universe?
Relating Science and Technology to Society and the Environment
  • What different types of resources are needed to realize projects like Breakthrough Starshot?
  • Why does the discovery of planets outside of our solar system matter to society?
  • How does the discovery of so many potentially habitable planets outside our solar system affect beliefs about intelligent life in the universe?
Exploring Concepts
  • How is the transit method used to detect exoplanets? 
  • Explain what it means to be tidally locked. How might this affect the potential living conditions on an exoplanet in the ‘Goldilocks zone’?
  • How might solar flares impact on a planet’s Goldilocks zone?
  • How is a red dwarf star similar or different to the Sun in Earth’s solar system? 
Exploring Concepts
  • How is the transit method used to detect exoplanets? 
  • Explain what it means to be tidally locked. How might this affect the potential living conditions on an exoplanet in the ‘Goldilocks zone’?
  • How might solar flares impact on a planet’s Goldilocks zone?
  • How is a red dwarf star similar or different to the Sun in Earth’s solar system? 
Nature of Science/Nature of Technology
  • What advances in technology have helped scientists to detect planets that they cannot even see? 
  • How important is the skill of creative thinking to the study of exoplanets? Explain.
Nature of Science/Nature of Technology
  • What advances in technology have helped scientists to detect planets that they cannot even see? 
  • How important is the skill of creative thinking to the study of exoplanets? Explain.
Media Literacy
  • Do you have a favourite space travel movie or book? Why do you think there is so much popular culture media on this topic? How have movies and books on the topic of space travel changed over time? What influences inform these fictional movies and books? 
Media Literacy
  • Do you have a favourite space travel movie or book? Why do you think there is so much popular culture media on this topic? How have movies and books on the topic of space travel changed over time? What influences inform these fictional movies and books? 
Teaching Suggestions
  • This article can be used for teaching and learning of Math & Physics, Space, and Engineering & Technology related to astronomical distance, space probes, planetary science and stars. Concepts introduced include planets, extrasolar planets, exoplanets, Venus, transit method, solar eclipse, telescopes, Milky Way, Goldilocks zone, solar flares, Proxima b, red dwarf star and tidally locked. 
  • Before reading this article, to help engage students prior knowledge, teachers could have students read the backgrounder Where Can We Find Asteroids and Comets?
  • Before reading the article, teachers could also have students use a Vocabulary Preview learning strategy to help them engage prior knowledge and learn new terms. Ready-to-use Vocabulary Preview reproducibles are available in [Google doc] and [PDF] formats. 
  • After reading the article, teachers could have students do a Concept Definition Web learning strategy for the concept of exoplanets. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 
  • To extend and reinforce student learning on the topic of detecting exoplanets, students could watch the video Exoplanets Explained (Johnson Exolab at CalTech).
Teaching Suggestions
  • This article can be used for teaching and learning of Math & Physics, Space, and Engineering & Technology related to astronomical distance, space probes, planetary science and stars. Concepts introduced include planets, extrasolar planets, exoplanets, Venus, transit method, solar eclipse, telescopes, Milky Way, Goldilocks zone, solar flares, Proxima b, red dwarf star and tidally locked. 
  • Before reading this article, to help engage students prior knowledge, teachers could have students read the backgrounder Where Can We Find Asteroids and Comets?
  • Before reading the article, teachers could also have students use a Vocabulary Preview learning strategy to help them engage prior knowledge and learn new terms. Ready-to-use Vocabulary Preview reproducibles are available in [Google doc] and [PDF] formats. 
  • After reading the article, teachers could have students do a Concept Definition Web learning strategy for the concept of exoplanets. Ready-to-use Concept Definition Web reproducibles are available in [Google doc] and [PDF] formats. 
  • To extend and reinforce student learning on the topic of detecting exoplanets, students could watch the video Exoplanets Explained (Johnson Exolab at CalTech).

Learn more

The Number Of Earth-Like Planets In The Universe Is Staggering (2017) 

Article from Forbes by Robert Frost on how scientists know there are so many other solar systems like ours in the universe.

Habitable Exoplanets Catalog (2019) 

Planetary Habitability Laboratory provides a list of exoplanets with rocky composition, where the planets fall in the "Goldilocks Zone", and a star chart with locations where more might exist.

Exoplanet and Candidate Statistics 

Information on exoplanet and possible exoplanets from NASA.

Possible 2nd Planet Spotted Around Proxima Centauri (2019)

Article by Mike Wall for Space.com detailing information about the exoplanet Proxima b.

Red Dwarfs Explained (2016)

Video (5:50 min.) by Kurzgesagt about what Red Dwarf stars are, and how they are made.

Tess Exoplanet Mission (2018)

Article by Rob Garner for NASA  explaining the Transiting Exoplanet Survey Satellite mission.

Breakthrough Starshot - Nanocraft to Alpha Centauri (2016)

Video (4:28 min.) from SciNews about using light beams to send ultra-light space probes that could reach Alpha Centauri in just over 20 years.

References

European Southern Observatory. (2010). Exoplanets press kit.

Howell, E. (2018, March 30). Exoplanets: Worlds beyond our solar system. Space.com.

Lissauer, J. J. (2018, September 10). Habitable zone. Encyclopaedia Britannica.

Sanders, R. (2013, November 4). Astronomers answer key question: How common are habitable planets. Berkeley University of California.

Stromberg, J. (2014, March 19). How do astronomers actually find exoplanets? Smithsonian Magazine.