Can Venus help us find exoplanets?…

This post is in response to loyal reader Jarman Day-Bohn’s question, which he left in a comment on the post “Today is transit day….”. Jarman asked:

How much do you think this [transit of Venus] will contribute to the current research experts are performing toward the study of possible earth-like planets out there? I know they were heavily using a technique measuring how much of a star’s light is blocked out by a planet to judge its size and other factors. Will this in any way help that process?

Great question Jarman, thanks for asking! Let’s look into that a little bit more.

But before we get too far into that, let’s think a little more about transits. Whether or not a transit occurs is all based on perspective. From Earth, only Mercury and Venus are interior planets- planets orbiting closer to the Sun- so they’re the only two that can we can see transit the Sun’s disk. If you were on Mars though, you could conceivably see Mercury, Venus, and Earth transits. And if you were on Pluto you could, in theory, see all eight planets transit the Sun. Remember though, as you get further away, even though more planets can be seen transitting the Sun from your perspective, you’re also getting further away, meaning the Sun is going to look smaller and smaller to you, as are the transitting planets. The Sun is only 93 million miles from the Earth (that’s really close astronomically speaking), so the enormous Sun, which is 1 million times larger in volume than the Earth, takes up a relatively large portion of the sky (~0.5 degrees). As you move further away from the Sun, its angular size in the sky will shrink. By the time you got to Pluto, which is 3.67 billion miles from the Sun, but still close astronomically speaking, our local star would look like a bright speck only ~0.01 degrees (50 times smaller than in the sky on Earth); probably something similar to the artist’s depiction below.

This artist’s depiction shows what the Sun might look like from an object, like Pluto, that’s in the solar system’s Kuiper Belt. Notice how the relatively close Sun differs from the background stars. Credit: NASA/JPL-Caltech/T. Pyle (SSC)

Now as you travel further from the Sun, let’s say to a planet orbiting another star, and look at our Sun, you could still in theory see all eight planets (and Pluto) transit the Sun, but now you’re trillions of miles from the Sun which is now just point of light in the sky, the same way other stars appear to us in the nighttime sky. The really astounding thing about looking at the other stars in our galaxy (Note: every star you see in the nighttime sky is in the Milky Way, we can’t resolve single stars in other galaxies.) is that they are so incredibly far away that no matter how large a telescope we use, we can never see the disk of the star like we can with the Sun, it just won’t have a large enough angular size. Now matter what, even through the Hubble Space Telescope, stars look like pinpricks of light that astronomers call “point sources“. Which makes actually seeing a planet transit a star, like we can see with Venus, impossible. I’ll remind you that in a previous post entitled “Baseballs, not umbrellas…” I explained the continuing search for exoplanets and covered all three of the main techniques which scientists employ to find these elusive celestial bodies around our galaxy. As Jarman indicated, the most successful and commonly used method of detection is the “transit method”. This is the method that NASA’s Kepler mission has already used to find the first Earth-like rocky exoplanet. However, since I just explained that we can’t actually see the exoplanet transitting the distant star, the only way we can detect the transit is by recording the change in light we see as the exoplanet crosses in front of the star. But again, we don’t actually “see” the transit happen, we just observe the dip in the brightness given off by the star. Similarly, if you were on a ship off the shore and someone walked in front of the lighthouse beacon, you wouldn’t be able to see the person, but you might be able to record the drop in brightness as they walked by.

This artist’s idea of NASA’s Kepler mission looking for exoplanets illustrates the main technique that scientists hope to use to find planets orbiting other stars- called the transit method- but no matter the size of the telescope we can’t actually see the exoplanets transitting the disk of the star. Credit: universetoday.com

But now let’s get back to Jarman’s actual question: can a transit in our solar system, like that of Venus, help scientists to find planets transitting other stars? It might. The transit of Venus is a well-documented and well-understood phenomenon, which scientists have been able to accurately predict and observe at least 6 times in the last 400 years. As I explained in “Looking to launch and preparing for transit…“, the transit of Venus helped us to determine the size of our solar system. And since we’ve actually been able to explore our solar system and have a very good grasp of the size of Venus and the Sun and the distances between the Earth and each of them, we can use the transit of Venus as a calibration tool in our search for extrasolar planets. For instance, scientist can say a planet like Venus, which is so big, transitting in front of a star like the Sun, which is so big, would cause a drop in brightness of this much at this distance. Then we can scale that distance out to other stars and we get some idea of what we need to look for in our search for exoplanets.

So Jarman, there’s your answer: while the spectacular crossing of Venus may not lead to groundbreaking new methods to find exoplanets, it does give us a rare opportunity to view a transit (that involves objects we know a lot about) and use that as a reference point as we continue our search!

Thanks again to Jarman for posing this question and if you, like Jarman, have a question that you’d like to have answered, please leave a comment or use the “Contact astroian” tab at the top of the page to send me an email!

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2 Responses to Can Venus help us find exoplanets?…

  1. Thank you Ian! You’re great at explaining these concepts to us in the drooling masses. Mr. Neil deGrasse Tyson couldn’t have done it better 🙂 Maybe you two should get a show together. I know you have the performance skills for it…

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