Possible errors in estimating the size of exoplanets in binary systems

Exoplanets and binary systems (Image NASA/JPL-Caltech)
Exoplanets and binary systems (Image NASA/JPL-Caltech)

An article to be published in the “Astronomical Journal” describes a study of possible errors made in the density estimates of the exoplanets discovered using the transit method. Elise Furlan of Caltech/IPAC-NExScl and Steve Howell of NASA’s Ames Research Center analyzed the effect of the presence of a companion star on estimating the density of exoplanets in those systems, concluding that some of them are actually less dense than calculated.

This problem is connected to the transit method used by some space telescopes to detect the existence of exoplanets. For example NASA’s Kepler Space Telescope uses that method and it’s the most important exoplanet hunter in this first phase of the history of the study of other solar systems. In just over twenty years of this type of activity scientists have accumulated a lot of experience but improvements are also being made to detect and correct errors.

In this specific case, the problem is that space telescopes can’t always differentiate between the presence of one or two stars in their observations. If two stars are close enough even the Kepler space telescope will see them as a single point. This could have significant consequences on the estimate of the characteristics of the exoplanets detected in those systems, a problem that could be widespread, given that binary systems are the norm.

The Tatooines, as exoplanets orbiting two stars are nicknamed, are the exception but binary systems can have exoplanets orbiting one of the two stars. There are cases where the position of the two stars compared to the Earth can deceive the telescope that’s being looking at them and it may happen that only further observations made with other instruments allow to understand the problem.

Everything begins with the passage of an exoplanet in front of its star. When a space telescope detects it, astronomers measure the apparent drop in the star’s brightness, which is directly proportional to the planet’s size. Its mass can be estimated through other methods and at that point it’s possible to estimate the planet’s average density, which indicates whether it’s rocky or gaseous.

If astronomers know that there are two stars in the observed system, they keep into account that their combined light is measured. However, if an exoplanet orbits one of the two stars, it will cause a decrease in its brightness only. The consequence is that if astronomers don’t know the presence of another star its size will be underestimated. The picture illustrates the issue.

Elise Furlan and Steve Howell studied 50 exoplanets previously examined thanks to the Kepler space telescope’s observations, whose radii and masses were estimated. They orbit stars that have a companion at distances not exceeding 1,700 times that between Earth and Sun. In 43 cases the estimates didn’t take into account the light emitted by the companion.

Actually, in 35 cases the error is limited. In 24 cases the exoplanet orbits the brighter of the two stars. There are, however, 15 cases in which the researchers failed to figure out whether the exoplanet orbits the fainter star but among them there are 5 in systems with two stars of almost equal brightness. These are cases where the error in size estimate and therefore average density can be substantial.

Understanding the characteristics of the exoplanets discovered accurately is important to improve our models of solar systems’ birth and evolution. Future space telescopes could provide more accurate observations but astronomers would want to choose at least some of their targets with some certainty.

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