Thirty years ago, scientists found the first exoplanets. Those are planets that orbit stars outside our solar system. Today, such distant worlds no longer seem rare. And now, with the latest cache of confirmed planets, the exoplanet count has passed 5,000.
Exoplanets have become so common that astronomers expect most stars to host at least one, says Aurora Kesseli. She’s an astronomer at the California Institute of Technology in Pasadena. And with so many exoplanets known, astronomers can start understanding their diversity, Kesseli says.
The 5,021 confirmed planets include gas giants, hulking planets made mostly of hydrogen and helium, similar to Jupiter and Saturn. They also include smaller terrestrial (rocky) planets like Earth, as well as “super-Earths” that are larger than Earth but smaller than Neptune and can be rocky or gassy. Finally, there are Neptune-like gassy planets that may be hot or cold.
Researchers have several ways to sniff out exoplanets. Some planets are so big that their gravity tugs on the star. This makes the star wobble. Scientists can look for that wobbling, a method called radial velocity measurement. Other planets can be spotted because they slightly dim the light of their parent stars when they pass in front of them. This is called the transit method. A third method, called microlensing, relies on the knowledge that planets’ gravity bends starlight. Lastly, imaging techniques can catch planets on camera.
Astronomers can’t say much about these worlds beyond diameter, mass and density. But several projects, such as the long-awaited James Webb Space Telescope, are working to change that. “Not only are we going to find tons and tons more exoplanets, but we’re also going to start to be able to actually characterize the planets,” Kessili told Science News.
The search is far from over. NASA’s newest exoplanet hunter, the TESS mission, has found thousands of planets that have not yet been confirmed. Ground-based telescopes keep adding to the count too. “There’s tons of exoplanets out there,” Kesseli says, “and even more waiting to be discovered.”
Table 1: Planet types
Exoplanet count (as of 5/9/2022): 5021
Table 2: Method of discovery
- Make a bar chart from the data in Table 1. Which type of planet is the most common? Which is the least common?
- About how many times more common are the most common type than the least common type?
- Add up the numbers in Table 2. Subtract that number from 100. What percent of planets were found by methods other than these four?
- Display these data (with your “Other” category) as a pie chart. You can do this in a computer program like Excel or Google Sheets. Or you can draw a circle and estimate how large a slice these four methods and other would take up.
- Can you think of other ways to show the data in these two tables, perhaps together?
- Go to NASA’s exoplanet Discoveries Dashboard. Scroll down until you reach the “Exoplanet census.” Move the red circle with arrows to 1989. Now watch as you move it slowly forwards in time to 2022. What do you notice about the four major methods to spot exoplanets? How does the rate of exoplanet discovery change over time?
- Now go to NASA’s Exoplanet Catalog. Pick a planet. When was it discovered? What type of planet is it? How far is it from Earth? Click on its link. What do scientists think this planet looks like? What questions do you have about this planet?