A Candidate Planet in the White Dwarf “Forbidden” Zone

Title: The MIRI Exoplanets Orbiting White Dwarfs (MEOW) Survey: Mid-Infrared Excess Reveals a Giant Planet Candidate around a Nearby White Dwarf

Authors:  Mary Anne Limbach et al.

First Author’s Institution: University of Michigan

Status: Accepted to The Astrophysical Journal Letters [open access]

Stars go through a natural lifecycle which mostly consists of steadily burning hydrogen into helium for tens of millions to billions of years. That’s exactly what our Sun is doing right now, making it a good time to be alive on Planet Earth. However, when our Sun runs out of hydrogen fuel, it will swell into a Red Giant. During this phase of its life, the Sun will grow so large that it will swallow up Mercury and Venus, and will likely extend to swallow up Earth, perhaps even Mars. This process is expected to destroy these inner planets. Ultimately at the last stage of stellar evolution, the Sun will shed its outer layers leaving behind just the core, which is now known as a White Dwarf star.

While the details of this final death dance are still hazy, most astronomers agree that planets within a certain distance of their dying host star should be destroyed and therefore we should not expect to find exoplanets close to a white dwarf host star. However, we must always allow for surprises and expect the unexpected because today’s Astrobite is precisely that: a planet candidate has been announced at a close enough separation from its white dwarf host that it should have been engulfed and destroyed. 

The MIRI Exoplanets Orbiting White dwarfs survey (MEOW for short, a rare excellent acronym in astronomy!) is currently carrying out observations of nearby white dwarf stars to search for planets. The field of exoplanets orbiting white dwarfs is still only a few years old, and so very little is known about the planets that orbit these ancient stars. The properties of white dwarfs make it extremely difficult to detect any nearby exoplanets. White dwarfs are in many ways the hardest kind of star to find an exoplanet. First, they are extremely small, only about the size of Earth, so even if there is an exoplanet in the system, the probability of it transiting such a small star is astronomically (sorry) smaller than the already small probability of it transiting a larger star. Furthermore, white dwarfs have almost no absorption features in their spectrum, so radial velocity observations are essentially impossible. 

But MEOW takes advantage of these undesirable features to search for planets in a new way: infrared excess. Because White Dwarfs are so small, and therefore dim, any additional light coming from the system, particularly at mid-infrared wavelengths where these stars emit almost no light, must be coming from some other body. It just so happens that planets, particularly giant gas planets, are brightest at these same red wavelengths. Therefore, if you find an more infrared light coming from a white dwarf than you expect, there is a chance that it is due to a planet. MEOW searches for this excess red light using JWST’s MIRI instrument, which is optimized to observe these red wavelengths.

Today’s Astrobite highlights the first major discovery from the MEOW survey. The authors announce that WD 0310-688, a white dwarf at only 10.4 parsecs away, essentially our backyard, has an infra-red excess that is consistent with a planet of 3 Jupiter masses orbiting between 0.1 and 2.0 AU from the star (see Figure 1). This is totally at odds with the expectation that there should not be planets in close orbits of white dwarfs hosts. 

This said, the team is cautious. Another explanation for the measured infra-red excess could be a debris disk, which white dwarfs are known to have. The team carefully weighs out the arguments in favor and against each possible scenario. 

First, the scenario that this is a Jupiter-like planet. 

In favor: a planet of this mass might be expected to survive the death of the star and the excess light is in line with coming from an object of Jupiter’s size. 

Against: the source of the excess light is very close to the white dwarf and this is expected to be very rare. In fact, the derived orbital parameters of this planet are within the zone where we would expect the planet to have been destroyed, although the planet may have migrated inward after the death of the star, as giant planets are known to do. 

Second, the scenario that this is a debris disk. 

In favor: disks are common and while this would be a colder disk than ever measured before, JWST is a better instrument than we have ever had before. Additionally, the white dwarf is polluted, meaning it likely has planetary-like material continually falling into it, potentially from a disk.

Against: In order to match the excess measured, the disk would have to be a very precise size, which is unlikely, and the disk would be further out from the star than other similar white dwarf disks.

The authors propose a test to break this tie: get a few hours of observation time with JWST on the target to get a spectrum. If this proves to be a planet, the implications are huge. It would be the coldest planet discovered that is amenable to direct spectroscopy through which we can learn more about the planet’s atmosphere and composition. Additionally, the precursor to the white dwarf was an A or B type star. It’s difficult to detect planets around these stars, with almost all known ones coming from direct imaging, which requires wide separations between the star and the planet. If this planet is confirmed, then it represents a rare example of a giant planet in close orbit to a former A/B star and probes a new area of parameter space.

In all, this is an incredibly exciting discovery from the MEOW team. We will continue to follow this system and this survey (it’s looking at 20 total nearby white dwarfs) and await more exciting results!

Astrobite edited by Nathalie Korhonen Cuestas

Featured image credit: Limbach et al. 2024