Future telescopes could see the wrong planets

We have discovered thousands of exoplanets in recent years, some the size of Earth and potentially habitable. But we don’t have seen many of these worlds. Most of the exoplanets we’ve found were discovered using the transit method, which involves watching a star’s luminosity plunge as a planet passes in front of it. We can learn the size and sometimes the mass of these hollows, but we have no idea what the world looks like, or if it has a breathable atmosphere.

Fortunately, this will change in the near future. New telescopes scheduled for launch in the next decade, such as the Nancy Grace Roman telescope, will be able to directly image Earth-sized exoplanets. But as a new study shows, that on its own will not be enough. We’ll also need to make sure we’re imagining the right planets.

It is very difficult to observe a planet directly. Compared to their star, they are small and dull, so their light can be masked by the glare of their sun. Astronomers have developed a few methods to block light from stars near a planetary image, such as the coronograph method, and as a result, we directly imaged a handful of exoplanets. These planets tend to look like Jupiter and orbit a good distance from their star. The Roman telescope will use more sophisticated methods, and astronomers have come up with advanced methods such as a lampshade to block starlight. So it’s only a matter of time before we can directly observe small planets orbiting near their star.

But being able to see more exoplanets will also make things more confusing. In this latest study, the team simulated how different types of planets might appear under direct imaging, and they found that very different types of planets could be misidentified. At a larger level, larger planets will tend to be brighter than smaller ones, and planets with a larger orbit will tend to appear farther from their star. But the luminosity of a planet depends not only on its size but also on its albedo. An icy world is much brighter than a smoky world.

For the eye sockets, there is a similar effect. If the orbit of a planet is flat from our point of view, then we will always see it at a good distance from the star. But if it has a peripheral orbit, the planet will usually be seen near the star. Due to these effects, a small near world may appear large and far away, while a large distant planet may appear small and near. When the team ran their simulations, they discovered that an Earth-like world could be mistaken for a Mercury-like world 36% of the time. It could be confused with a planet similar to Mars 43% of the time and a planet similar to Venus 72% of the time.

This means that future surveys of exoplanets will not be able to capture a few images and move on. It will take a long series of observations to confirm the orbits and sizes of the exoplanets that we can see. Astronomers will need to be careful not to find themselves in a land of confusion.

Reference: Keithly, Dean Robert and Dmitry Savransky. “The solar system as an exosystem: planetary confusion”. Letters from the astrophysical journal 919.1 (2021): L11.