The ultimate fate of the Earth is pretty clear. In about 7 billion years the Sun will swell up into a red giant and swallow the Earth. Our planet will exist inside the Sun for some amount of time, and the results won’t be great. Earth won’t likely vaporize completely, but it won’t be a fun place to live, either.

This fate is inevitable for a lot of planets in our galaxy, if they orbit stars like the Sun and are too close in. As their host stars expand at the end of their lives they’ll take those planets with them. Not great.

We see evidence for such things in stars, too. Some planetary nebulae, the expanding shells of gas thrown off by a star while it’s a red giant, can have all sorts of weird shapes that are caused by gas giant planets that were consumed and stirred up the outer layers of the star. We see odd spectra of dead white dwarf stars, too, indicating they ate their planets. But these are indirect detections. 

So, in 2020, when a star about 14,000 light-years from Earth suddenly flared up and got much brighter than usual, astronomers got excited that this was the first direct detection of such an event. Seen by the Zwicky Transient Factory observatory, the event was labeled ZTF SLRN-2020. SLRN stands for Subluminous Red Nova; a nova is a spectacular brightening of a star, but this one wasn’t as bright as you’d expect for a regular nova, and the light it gave off was very red. It also didn’t have the bright gaseous spectral features a nova usually displays. You do see this sort of thing when two stars merge; the event is extremely energetic and creates a lot of dust (tiny grains of rocky or sooty material). The dust absorbs blue light, making the merged star look really red.

However, SLRN-2020 didn’t get nearly as bright as a merger would generally be, which is a really luminous event. The energy it gave off, though, was what’s expected for a star swallowing a planet as it expanded. The event was also bright in the infrared, which is expected due to the amount of dust generated as the plant vaporizes in the heat of the red giant.

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But mysteries remained. The star didn’t seem massive enough to have lived long enough to use up all its nuclear fuel, which is what triggers the red giant expansion. Also, the behavior of the star when seen in infrared light didn’t necessarily jibe with the star expanding and eating the planet. 

So astronomers pointed JWST at the star, hoping to resolve some of these problems. Turns out they did solve them, but not the way everyone expected. [link to journal paper].

The star didn’t eat the planet. The planet did a death dive into the star.

The observations were made about 2.5 years after the event occurred, and they found two major things. The first is that the brightness of the star didn’t make sense, and there appeared to be two different structures emitting infrared light.

The first bit is critical. Given the distance to the star, they could calculate how much energy it was emitting in the infrared. Red giants freaking blast out IR, but this star wasn’t doing that. In fact, the light was more consistent with a low-mass star, smaller than the Sun! Stars like this are cooler in temperature and therefore red, but they’re small. They found the star to likely be only about 70% the mass of the Sun.

The thing is, a star that lightweight uses its nuclear fuel very slowly, so it lives a long time. This star in particular has a lifespan longer than the current age of the Universe, which means it literally could not be a red giant! That in turn means it didn’t expand and swallow the planet. The planet must have somehow dropped down and collided with the star.

That can happen if one planet gets too close to another, and their gravitational interaction drops one very close to the star in a highly elliptical orbit. However, the observations don’t support that; over time the infrared light from the star grew steadily and slowly, which you don’t expect for a planet dipping close to the star once per orbit. The observations make more sense if the planet is instead orbiting the star very closely on a circular orbit. So, it may have started off on an elliptical orbit after encountering another planet, but then gravitational interactions with the star caused the planet to slowly spiral in close. As it did so it heated up and lost material, adding steadily to the infrared light emitted.

This means the planet was on a very tight orbit. We see planets like this; in fact the first planets discovered orbiting Sun-like stars were gas giants on incredibly close orbits to their stars, some mere millions of kilometers above the surface, far closer than Mercury is to the Sun! These are called hot Jupiters.

What happens next depends a lot on the density of the planet compared to the star. Red giants are puffy, and low density, whereas Jupiter has a density similar to water. It can survive a long time because it can hold itself together, so it dies slowly, losing material from its upper atmosphere as it heats up.

But a normal low-mass star is actually denser than a gas giant. That means the tides from the star can disrupt the planet just before it falls in, literally tearing it apart. Planets are very hot inside, and this is a very energetic event. All that heat is released, and you get essentially a big explosion of energy. The star becomes a lot more luminous, and more infrared light is emitted (just as was seen in SLRN-2020).

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