With help from NEOWISE, astronomers observe giant star destroying planet for the first time

by Haygen Warren

Using the California Institute of Technology’s (Caltech) Zwicky Transient Facility (ZTF) and NASA’s Near-Earth Object Wide Field Infrared Survey Explorer (NEOWISE) observatory, a group of scientists has observed an aging star destroying a planet within its own star system for the first time. Astronomers have long believed that such a phenomenon occurred in red giant star systems, but have never directly observed the event — until now.

Given that our own Sun will evolve into a red giant star in approximately five billion years, the research, led by Kishalay De of the Massachusetts Institute of Technology in Cambridge, Massachusetts, will provide astronomers with a glimpse into what the future of our solar system may look like when our Sun eventually absorbs Mercury, Venus, and possibly Earth.

“This type of event has been predicted for decades, but until now we have never actually observed how this process plays out,” De said.

When a main-sequence star, or any star that fuses hydrogen into helium within a hot, dense core, reaches the later years of its life, it will eventually run out of fuel within its core. When this happens, the star will begin to dramatically increase in size and turn more reddish in color — earning the name “red giant.” If this red giant has planets orbiting near it, those planets will typically be swallowed by the red giant as it expands in size, destroying the planets.

As mentioned, astronomers have long theorized that red giants absorb and destroy planets within their own system, but have never been able to directly observe the phenomenon with telescopes. However, using ZTF, NEOWISE, and a number of other ground-based observatories, De et al. were able to collect data on such an event and even determine the characteristics of the planet that was destroyed by its host star.

In the specific event studied by De et al., named ZTF SLRN-2020, the planet was roughly the same size as Jupiter. However, the orbital distance of the planet in ZTF SLRN-2020 was much, much shorter than the orbital distance of Jupiter from our Sun — orbiting closer to its host star than Mercury orbits our Sun.

Furthermore, the NEOWISE and ground-based observatories’ data allowed De et al. to predict the sequence of events that occurred during the death of the planet.

As the star began to expand into the orbital plane of the planet, the atmosphere of the red giant surrounded the planet. The drag produced by the planet entering the star’s atmosphere substantially slowed the planet’s orbital speed, causing its orbit to shrink and sink even lower into the star — similar to how objects enter Earth’s atmosphere.  The energy transfer from the planet entering the star temporarily caused the star to dramatically increase in size and brightness, with its brightness increasing by several hundred times its usual magnitude. The star then returned to its normal size and magnitude.

However, how did De find ZTF SLRN-2020 and confirm that it was a planet being absorbed and destroyed?

Interestingly, De wasn’t actually searching for planets being destroyed by their host stars — instead, he was searching for novae, a phenomenon in which a white dwarf cannibalizes hot gas from other nearby stars. Using ZTF, a facility located at the Palomar Observatory in Southern California that searches for objects that rapidly change brightness, De noticed a bright flash of light from a star.

De originally assumed that this bright flash was just another novae. However, this would prove to be false.

Novae are typically surrounded by massive flows of hot gas, meaning they tend to appear very bright to telescopes and other observational facilities. However, follow-up observations of the flash noticed by De showed that the gas and dust surrounding the star were very cool — characteristics that are not common in novae. De had never seen anything like it and immediately began planning for future observations of the star.

One such follow-up observation utilized NEOWISE, an observatory that observes the entire sky in infrared light every six months, creating sky maps that allow astronomers to observe how objects change over time. From the NEOWISE data, De found that the brightening was evidence of dust, which can emit light in infrared, being produced by and circulating around the star. What’s more, the NEOWISE data showed the star brightening nearly a year before ZTF observed the flash.

“Very few things in the universe brighten in infrared light and then brighten in optical light at different times. So the fact that NEOWISE saw the star brighten a year before the optical eruption was critical to figuring out what this event was,” De explained.

De et al. believe the dust surrounding the star after ZTF SLRN-2020 indicates that the planet fought until the very end, as the planet likely pulled hot gas away from the star as it entered its atmosphere. The hot gas that was pulled from the star would have cooled and turned to dust. This is likely the dust seen in the NEOWISE observations. Furthermore, as the planet fell deeper into the star and began to be pulled apart, even more of the star’s hot gas would’ve been ejected away from the star, producing more dust.

Artist’s illustration of NEOWISE. (Credit: NASA/JPL-Caltech)

As was mentioned, the De et al. data may give scientists insight into how our solar system may look five billion years from now when the Sun expands and swallows Mercury, Venus, and, possibly, Earth. However, as De explained, the event would not be nearly as noticeable as ZTF SLRN-2020 due to the small sizes of Mercury and Venus.

“If I were an observer looking at the solar system five billion years from now, I might see the Sun brighten a little, but nothing as dramatic as this, even though it will be the exact same physics at work,” said De.

Astronomers currently believe that a handful of red giants consume planets within our galaxy each year. De et al.’s research will help create a template for future studies that plan to investigate the absorption and destruction of planets by red giants.

“This discovery shows that it’s worthwhile to take observations of the entire sky and archive them, because we don’t yet know all of the interesting events we might be capturing. With the NEOWISE archive, we can look back in time. We can find hidden treasures or learn something about an object that no other observatory can tell us,” said NEOWISE deputy principal investigator Joe Masiero of the Infrared Processing and Analysis Center at Caltech.

De et al.’s research was published on May 3 in the journal Nature.

(Lead image: artist’s concept of ZTF SLRN-2020. Credit: R. Hurt/K. Miller (Caltech/IPAC))

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