Astronomers Watch a Star Gulp Down One of its Planets

A star like our Sun only shines the way it does because of its intrinsic balance. Stars are massive, and the inward gravitational pressure from all that mass acts to contain the outward thermal pressure from all the fusion inside the star. They are in equilibrium, or on the main sequence if you like, and the result is a spherical mass of plasma that holds its shape and emits radiation with relative stability for billions of years. Like our Sun.

But eventually, stars teeter over the edge and lose their balance. Stars like our Sun will expand, take on a malevolent red hue, and begin to destroy anything that comes within their grasp.

Like a planet.

As a star like our Sun fuses hydrogen into helium for billions of years, it loses mass. As it loses mass, its inward force of gravity weakens. Eventually, gravity weakens so much that it can no longer counterbalance all the outward pressure from fusion. It cools, turns red, and expands. It becomes a red giant.

Astronomers know all about this because when they look out into the galaxy with powerful telescopes, they can observe and study stars in all stages of life. They also know that our Sun will follow this path. Eventually, it’ll become a red giant and expand. It’ll consume and destroy Mercury, Venus, and most likely Earth too. It’s an inescapable fate. There’s no technological tool we can wield to save Earth.

For us, this is way in the distant future, billions of years from now. Maybe our distant descendants, if we have any, will escape to another planet or moon or crowd into a generational starship and keep humanity going somehow. If that happens, then those humans, if that’s what they still are, will look back ruefully at the wreckage that used to be the inner planets.

That’s a dramatized version of what will happen because, in actual fact, the Sun will slowly heat up well before it expands and becomes a red giant. It’ll boil Earth’s oceans away, shred Earth’s atmosphere, and sterilize the planet. It won’t be cinematic; it’ll happen over a long period of time.

But any way you look at it, a star consuming a planet is a dramatic event. Astronomers at MIT, Harvard, CalTech, and other institutions caught a glimpse of this drama when they saw a distant red giant consuming one of its planets about 12,000 light-years away in the Eagle (Aquila) constellation. Even though red giants aren’t rare, and they’re likely consuming and destroying planets throughout the Milky Way, this is the first time astronomers have spotted it happening.

“We were seeing the end-stage of the swallowing.”

Kishalay De, MIT Kavli Institute

It all started with the Zwicky Transient Facility (ZTF), a wide-angle camera on one of the telescopes at the Palomar Observatory in California. The ZTF is an automated survey facility that images the entire northern sky every two nights. It’s monitored by astronomers who are interested in different types of transients. The ZTF detects transients like supernovae, flare stars, and asteroids in a field called time-domain astronomy, or the study of how astronomical objects change over time.

In May 2020, the ZTF spotted a star that grew brighter by over 100 times in only ten days, then quickly faded again. When something brightens that much, it’s typically a supernova or something similar. But this one was different. After its rapid brightening, there was a colder, longer-lasting signal. According to the team of researchers, only one event can produce this signal: a star devouring a planet.

The researchers presented their findings in a paper titled “An Infrared Transient from a Star Engulfing a Planet” in the journal Nature. The lead author is Kishalay De, a post-doc at MIT’s Kavli Institute for Astrophysics and Space Research.

“We were seeing the end-stage of the swallowing,” De said in a press release announcing the findings.

“It was unlike any stellar outburst I had seen in my life.”

Kishalay De, MIT Kavli Institute

According to the team, this is what they witnessed.

Lead author De was engaged in different research when it happened. He was searching for eruptions in stellar binaries, stars that orbit each other so closely that one draws matter from the other. The matter transfer is variable, and when it happens, the star receiving the matter brightens temporarily. This is one of the types of transients that ZTF is tuned to detect.

Astronomers studying particular types of stellar objects are accustomed to seeing certain patterns in the light the objects emit. The types and amount of light they emit over time give things like stellar binaries tell-tale light curves. The stellar binaries De was studying are called Luminous Red Novae (LRN), but the light signals from the event, now named ZTF SLRN-2020, didn’t match those from any LRN De had seen before.

So when astronomers detect a light curve that is unlike anything else they’ve observed, excitement builds.

“One night, I noticed a star that brightened by a factor of 100 over the course of a week, out of nowhere,” De recalls. “It was unlike any stellar outburst I had seen in my life.”

This figure from the study shows how ZTF SLRN-2020 (green squares top; red circles bottom) compares to other LRN. The key takeaway is that it brightened much more rapidly than other LRN. Image Credit: De et al. 2023.

When astronomers confront something like this, their next step is to look at it with different tools to see what they can learn. In this case, De turned to the Keck Observatory in Hawaii. While the ZTF is a powerful tool for spotting objects that change their brightness, it can’t determine much detail. But the Keck Observatory is different. The Keck Telescopes can take spectroscopic measurements of light, which can reveal a star’s chemical composition.

At this point, De was still working with the assumption that this was a stellar binary, the objects he was engaged in studying. But Keck’s spectroscopic measurements showed that something different was happening. While binaries typically give off hydrogen and helium as they interact, this new source gave off neither. Rather than simple hydrogen and helium, spectroscopy showed the presence of ‘peculiar molecules.’ These molecules can’t exist in a hot, binary star environment. They can only exist in colder temperatures.

“These molecules are only seen in stars that are very cold,” De says. “And when a star brightens, it usually becomes hotter. So, low temperatures and brightening stars do not go together.”

The presence of these molecules meant that what De was seeing could not possibly be a stellar binary. De waited for more information on the event. About a year after it was first discovered, De and some of his colleagues gathered infrared observations. Was it really cold enough for De’s ‘peculiar molecules’ to exist?

This figure from the study shows some spectra from ZTF SLRN-2020 as well as the spectra of two other objects for comparison. The Palomar Observatory infrared observations are in the black line labelled P200 for the Palomar 200-inch telescope. The presence of VO (vanadium oxide,) TiO (titanium oxide,) and others signals much cooler temperatures than exist at a stellar binary. Image Credit: De et al. 2023.

At this point, the researchers were still wondering if what they were seeing could be a supernova. But as more observational data came in, it became apparent that it wasn’t. ZTF SLRN-2020 had an initial hot flash, but more infrared observations showed that the event was emitting colder energy over the next year. The stellar binary was emitting gas that condensed into cold dust, cold enough that infrared detectors could sense it. That eliminated a supernova as a cause but not a luminous red nova.

“That infrared data made me fall off my chair,” De says. “The source was insanely bright in the near-infrared.”

That’s when we realized: This was a planet crashing into its star.”

Kishalay De, MIT Kavli Institute

The team of researchers kept analyzing the existing data and added new infrared data from NASA’s Neowise space telescope. They were able to characterize the total energy released by ZTF SLRN-2020 since its initial outburst in 2020 and were in for another surprise. The amount of energy released was shockingly small and far too small for any other stellar mergers observed in the past: only about 1/1000th.

“That means that whatever merged with the star has to be 1,000 times smaller than any other star we’ve seen,” De says. “And it’s a happy coincidence that the mass of Jupiter is about 1/1,000 the mass of the sun. That’s when we realized: This was a planet crashing into its star.”

At this point, the puzzle was complete. ZTF SLRN-2020’s initial hot, bright flash was a planet with the same mass as Jupiter being dragged into the expanding atmosphere of a red giant star. This is a cataclysmic event, and as the star consumed the planet, material from the star’s outer layer was blasted away into space. Over the next year, all that gas condensed into cold dust, which explains the infrared data that was such a surprise to De.

This whole event is a first in astronomy. We know that as stars like our Sun become expanded red giants, they must eat planets that get too close. Astronomers know this because they’ve seen the before and after. But they’ve never caught a star in the act before.

“For decades, we’ve been able to see the before and after,” De says. “Before, when the planets are still orbiting very close to their star, and after, when a planet has already been engulfed, and the star is giant. What we were missing was catching the star in the act, where you have a planet undergoing this fate in real time. That’s what makes this discovery really exciting.”

While every new discovery in astronomy is interesting and exciting in its own way, this one has a melancholy feel to it.

When this happens in our Solar System, Earth will first cease to be habitable and then will be destroyed. Humanity’s home will be gone. The planet consumed by ZTF SLRN-2020 is far too massive to be Earth-like and is almost definitely a gas giant, so there’s likely no way it ever supported life. But if it had smaller siblings, like in our Solar System, it’s within the realm of possibility that those other, smaller planets were already destroyed. While we’ll never know for certain if there were other planets more similar to Earth, it’s possible that there may have been life there, even simple, single-celled life.

If there was, all that’s left is a puff of cold dust.


Evan Gough

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