Future Telescopes Could be Seeing the Wrong Planets

We have discovered thousands of exoplanets in recent years, including some that are Earth-sized and potentially habitable. But we haven’t seen many of those worlds. Most of the exoplanets we’ve found have been discovered using the transit method, which involves watching the brightness of a star dip as a planet passes in front of it. We can learn the size and sometimes the mass from these dips, but we have no idea what the world looks like, or whether it has a breathable atmosphere.

Fortunately, that will change in the near future. New telescopes scheduled to be launched 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 by itself won’t be enough. We’ll also have to make sure we’re imaging the right planets.

We have only imaged a few planets directly, such as TYC 8998-760-1 b and c seen here. Credit: ESO/Bohn et al

It’s very difficult to observe a planet directly. Compared to their star, they are small and dim, so their light can be hidden by the brilliance of their sun. Astronomers have developed a few methods to block nearby starlight from a planetary image, such as the coronagraph method, and as a result, we have directly imaged a handful of exoplanets. These planets tend to be Jupiter-like and orbit a good distance from their star. The Roman telescope will use more sophisticated methods, and astronomers have proposed advanced methods such as a starshade to block starlight. So it’s only a matter of time before we will directly observe small planets orbiting close to 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 broad 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 brightness of a planet depends not only on its size but also its albedo. An icy world is much brighter than a charcoal one.

Images of Enceladus, the Earth, the Moon, and Comet 67P/C-G, with their relative albedos, scaled correctly. Credit: ESA’s Rosetta Blog

For orbits, there is a similar effect. If a planet’s orbit is flat-on from our point of view, then we will always see it a good distance from the star. But if it has an edge-on orbit, the planet will usually be seen close to the star. Because of these effects, a small, close world might appear large and distant, while a large distant planet might appear small and close. When the team ran their simulations, they found that an Earth-like world could be confused with a Mercury-like world 36% of the time. It could be confused with a Mars-like planet 43% of the time and a Venus-like one 72% of the time.

What this means is that future exoplanet surveys won’t 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 we can see. Astronomers will need to be careful so that they don’t end up in a land of confusion.

Reference: Keithly, Dean Robert, and Dmitry Savransky. “The Solar System as an Exosystem: Planet Confusion.” The Astrophysical Journal Letters 919.1 (2021): L11.