Astronomers Have a New Way to Find Exoplanets in Cataclysmic Binary Systems

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Have you heard of LU Camelopardalis, QZ Serpentis, V1007 Herculis and BK Lyncis? No, they’re not members of a boy band in ancient Rome. They’re Cataclysmic Variables, binary stars that are so close together one star draws material from its sibling. This causes the pair to vary wildly in brightness.

Can planets exist in this chaotic environment? Can we spot them? A new study answers yes to both.

Cataclysmic Variables (CVs) undergo large increases in brightness. All stars vary in brightness to some degree, even our own sun. But CVs increased brightness is much more pronounced than stars like our Sun, and they happen irregularly.

There are different types of cataclysmic variables: classical novae, dwarf novae, some supernovae, and others. All types share the same basic mechanic. A pair of stars orbit each other closely, and one of the stars is more massive than the other. The more massive one is called the primary star, and it draws gas from the lower mass star, which astronomers call the donor star. The primary star in a CV is a white dwarf, and the donor star is usually a red dwarf. The red dwarf stars are cooler and less massive than the white dwarfs. They have masses between 0.07 and 0.30 solar masses and a radius of about 20% of the Sun’s. White dwarf primary stars have a typical mass of around 0.75 solar masses but much smaller radii, about the same as Earth’s.

When the primary star draws material from the donor star, the material forms an accretion disk around the primary star. The material in the accretion disk heats up, and that causes increased luminosity. The increase can overpower the light from the pair of stars. If there’s a dim third body—a planet—in the system, then its gravity can affect the transfer of material from the donor to the primary star. These perturbations affect the system’s brightness, and that’s at the heart of the new study.

This is an image of the cataclysmic variable AE Aquarii, which isn't part of this study. The smaller yet more massive white dwarf is drawing material away from its main-sequence companion, usually a red dwarf. Image Credit: By Casey Reed / NASA - Nasa - White Dwarf Pulses Like a Pulsar, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4282713
This is an image of the cataclysmic variable AE Aquarii, which isn’t part of this study. The smaller yet more massive white dwarf is drawing material away from its main-sequence companion, usually a red dwarf. Image Credit: By Casey Reed / NASA – Nasa – White Dwarf Pulses Like a Pulsar, Public Domain, https://commons.wikimedia.org/w/index.php?curid=4282713

The authors of the study show how the chaotic environments around CVs can host planets and explains how astronomers can spot them. The study is “Testing the third-body hypothesis in the cataclysmic variables LU Camelopardalis, QZ Serpentis, V1007 Herculis and BK Lyncis.” It’s published in the Monthly Notices of the Royal Astronomical Society (MNRAS.) The lead author is Dr. Carlos Chavez, from the Universidad Autónoma De Nuevo León in Mexico.

Material drawn to the primary star gathers in an accretion ring and heats up, creating increased luminosity. But the material transfer to the disk isn’t steady; it rises and falls as the stars in the CV orbit each other. Chavez and his colleagues examined four cataclysmic variables in their study: LU Camelopardalis, QZ Serpentis, V1007 Herculis and BK Lyncis. The four CVs exhibit very long photometric periods (VLPPs), which are periods of enhanced luminosity that don’t conform to the binary’s orbital periods.

There’s a point between both stars and the third body called the L1 point, or Lagrangian One point. It’s a gravitational equilibrium point between the stars. The L1 point is dynamic, and its position changes as the stars move. Lead author Chavez showed in a previous paper that a third body, a planet, can cause oscillations in the L1 point.

As the L1 point changes, the amount of material drawn into the primary star—the mass transfer rate—changes. A change in the mass transfer rate creates a change in the luminosity of the entire three-body system.

By measuring the changes in brightness of the four CVs, the researchers calculated the distances and masses of potential third bodies in the systems based on the brightness changes in each system. Their calculations show that the variations have much longer periods than the orbital periods of the stars. According to the team, two of the four CVs they studied have “bodies resembling planets” orbiting them.

“Our work has proven that a third body can perturb a cataclysmic variable in such a way that can induce changes in brightness in the system,” Dr. Chavez said in a press release. “These perturbations can explain both the very long periods that have been observed – between 42 and 265 days- and the amplitude of those changes in brightness. Of the four systems we studied, our observations suggest that two of the four have objects of planetary mass in orbit around them.”

This isn’t the first time scientists have tackled CVs and tried to find an explanation for the variations in the luminosity. In 2017 a separate team of researchers published a paper presenting the four CVs and their VLPPs. They suggested that planets were the cause. But they said that “… the third-body orbital plane should be greater than 39.2 degrees for this mechanism to be effective in disturbing the inner binary effectively.”

“Here we explore a new possibility, namely that the secular perturbation by a low eccentricity and low inclination third object explains the VLPP and also the change of magnitude observed in these four CVs,” Chavez and his co-authors write in their paper. They say that “… a third body on a close near-circular planar orbit could produce perturbations on the central binary eccentricity.”

According to Chavez, their work amounts to a new way to detect exoplanets. Planet-hunters find most exoplanets using the transit system. As an exoplanet transits in front of its star, there’s a detectable dip in starlight. While effective—we’ve found thousands of planets this way—the transit method has limitations. It only works when things are lined up right. We have to be looking at it from the side, as it were, or else the planet doesn’t transit the star from our point of view, and there’s no dip in starlight.

But the method Chavez and his colleagues developed doesn’t depend on planetary transits. It relies on the intrinsic change in luminosity that’s observable from different angles.

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