Stealth Binary Stars Caught out by Tattle-tale Exoplanets

Title: Do anomalously-dense hot Jupiters orbit stealth binary stars?

Authors: Tanvi Goswamy, Andrew Collier Cameron, Thomas G. Wilson

First Author’s Institution: Centre for Exoplanet Science, SUPA School of Physics and Astronomy, University of St Andrews, St Andrews, Fife KY16 9SS, UK

Status: Accepted to MNRAS [open access]

With the boom in exoplanet detection over the past few decades, we learn more and more about worlds outside our own Solar System. Some of the planets we have discovered have even been found to be part of a stellar system with multiple stars. The majority of the stars in our galaxy have one or more stellar companions, and most stars host planets, so the intersection of these populations isn’t a massive surprise. Worlds with two suns make for picturesque sci-fi scenes, but figuring out if a planet actually orbits more than one star isn’t always as simple as seeing two sunsets over a barren desert (or on a movie screen).

A good starting point for finding an exoplanet with two stellar neighbors is to find an exoplanet around one star first. One of the main methods for detecting exoplanets is detecting the light blocked by a planet shining in front of a star as it passes between its host star and Earth. This is called the transit method. In the case where the planet orbits two stars, but only one of them (the ‘primary’ star) has been directly detected, there would be additional light from the secondary star contaminating the measurement. This additional brightness makes the fraction of light lost due to the planet passing in front of the star(s) seem smaller, therefore the planet appears to have a smaller radius than it really does. However, we can still measure the primary star’s true radius, as this is given by the length of time of the planet’s transit, which is not changed by the light from the secondary star.

There is another way to measure the radius of the primary star, which relies on the relationship between the infrared flux, bolometric luminosity, the stellar temperature, and the distance to the star, giving an independent measurement of the stellar radius. This Infrared Flux method (IRFM) will give an incorrect measurement of the stellar radius if there is a second star contaminating the measurement, and so if the IRFM stellar radius disagrees with that given by the transit method, we know something fishy is going on… Today’s authors compare exactly these two methods, identifying exoplanets that live in binary systems by finding discrepancies in the measurements of their stellar radii. They look at a sample of exoplanets identified with the Wide-Angle Search for Planets (WASP) project, which has discovered over 178 gas giants in close orbits to their host stars via the transit method. The comparison of the two radii measurements for these planets is shown in Figure 1, showing eight exoplanets with significant disagreement in their stellar radii measurements. Furthermore, half of these outliers also have high renormalised unit weight error (RUWE) values, a metric that indicates a possible extended or binary source in the measurements – further evidence for stealth binaries!

Three of these exoplanets already had an identified binary star from previous work – showing that this discrepancy was a fair check. The other candidates appear to have so far evaded any detection of binarity. Any pair of stars that are very close together should orbit each other fast enough to cause some distinguishable Doppler shift in their spectra, and any that are far enough apart should be able to be seen as separate light sources in the imaging. This leaves a sweet spot of stellar binaries with periods around one hundred to several thousand years that are yet undetected. The authors show in Figure 2 this sweet spot in terms of angular separation and radial velocity difference, and that all of the WASP binary candidates lie within this region, assuming their orbital periods are in a range from 64 to 15000 years, explaining why these binaries have remained stealthy until now.

Now knowing these exoplanet hosts are not one but two stars, the authors recalculate the stellar and planetary parameters for the system – the true masses, radii, and densities of both stars and the planet. For one of the systems, WASP-20, it was found that the planet is orbiting the less massive, fainter star. Usually, we would expect to preferentially identify planets around the brightest stars, as they are easier to detect, so this unusual detection is exciting! 

Overall, accounting for the stellar binary causes the mass and the radius of the planet measurement to increase, while the density decreases. Therefore, these worlds with two suns are bigger but fluffier planets than we first expected. Knowing the true properties of these planets will help us understand better how these planets formed and got to where they are, how this was influenced by the second star in the system, contributing to the stories of all planets with two suns, even if one is stealthy.

Astrobite edited by William Lamb

Featured image credit: ESO/L. Calçada, modified by Storm Colloms

• About the Author

About Storm Colloms

Storm is a postgraduate researcher at the University of Glasgow, Scotland. They work on understanding populations of binary black holes and neutron stars from the gravitational wave signals emitted when they merge, and what that tells us about the lives and deaths of massive stars. Outwith astrophysics they spend their time taking digital and film photos, and making fun doodles of their research.

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