Are Hot Jupiters Tidally Disrupted During Stellar Main Sequence?

Imagine the Milky Way not just as a sea of stars, but as a bustling cosmic city. In this city, stars are like cars on a highway. When a car is brand new, it drives smoothly and stays close to the center lane. But as a car gets older, it starts to get bumped by other cars, swerve a bit more, and drift further out toward the edges of the road. Astronomers have figured out that the "wobble" or speed variation of a star (specifically how much it jiggles up and down relative to the flat disk of the galaxy) is a reliable clock. The more a star jiggles, the older it is.

Now, picture a special type of planet called a "Hot Jupiter." These are giant planets, similar in size to Jupiter, but they orbit their stars so closely that they are practically baking. Because they are so close, they act like a cosmic anchor, pulling on their host star with massive gravitational tides. The big question for scientists has been: Do these planets get destroyed by their stars before the stars get old? If they do, then the stars that still have Hot Jupiters should be the "young" ones—the ones that haven't had enough time to lose their planets yet. If this were true, we should find that stars with Hot Jupiters are "younger" (jiggle less) than other stars of the same type.

This paper takes a fresh look at that idea. The authors, a team of astronomers, decided to check the "jiggle" of stars hosting these planets to see if they are indeed younger than their neighbors. They found that a previous study had made a few mistakes in how they measured the stars' movements and which stars they compared. When the team fixed those mistakes and looked at the data with fresh eyes, the story changed completely. Instead of finding a crowd of young, jiggle-free stars with Hot Jupiters, they found that these stars jiggle just as much as the other stars around them. In other words, the evidence that Hot Jupiters are being swallowed up by their stars during the star's main life is much weaker than we thought. It seems these planets might be sticking around longer than the "tidal destruction" theory predicted, or at least, we can't prove they are disappearing just by looking at how fast their stars are wiggling.

The Cosmic Speed Bump

To understand what the authors did, we first need to understand the "Age-Speed" rule. In our galaxy, stars are born in a thin, flat disk. Young stars are like fresh drivers; they stay in a tight lane and don't move much up or down. As they age, they get bumped by giant molecular clouds and other stars, gaining energy and moving more chaotically. This creates a "vertical velocity dispersion" (a fancy way of saying the average amount of up-and-down wobble). The more the star wobbles, the older it is.

The previous study (known as HS19) looked at stars with Hot Jupiters and found they wobbled less than other stars. They concluded: "Aha! These stars are young! That means the Hot Jupiters are being destroyed quickly, so only the young stars still have them." It was a clever idea, but the new paper suggests the previous team might have been looking at the wrong speedometer.

The Mistake in the Map

The authors of this new paper, led by Qingru Hu and Wei Zhu, realized the previous study had a few "glitches" in their map. First, they used the total speed of the stars (how fast they moved in all directions) instead of just the vertical wobble (up and down). The vertical wobble is a much better clock for age. Second, the previous study mixed together stars found by different methods: some were found by looking for dips in starlight (transits), some by listening to the star's wobble (radial velocity), and some from space telescopes. These different methods found stars in very different parts of the galaxy.

Imagine trying to figure out the average age of people in a city by mixing a group of teenagers from a high school with a group of retirees from a nursing home, but you only looked at the teenagers who lived in the city center and the retirees who lived in the suburbs. If you didn't account for where they lived, you might get a confused result. Similarly, the "jiggle" of stars changes depending on where they are in the galaxy. The previous study didn't account for this, so their "young" stars might just have been stars from a part of the galaxy where everyone jiggles less naturally.

The Great Re-Check

The team decided to fix the map. They took the list of Hot Jupiter stars and cleaned it up, removing any that didn't fit the rules or were too far away to measure accurately. They then created a "control group" of normal stars that were the exact same age, color, and distance as the Hot Jupiter stars. This is like finding a twin for every Hot Jupiter host star, but without the planet.

When they compared the "jiggle" (vertical velocity dispersion, or $\sigma_W$) of the Hot Jupiter stars to their twins, the difference vanished.

• The Old Result: Hot Jupiter stars were much "younger" (lower jiggle).
• The New Result: Hot Jupiter stars jiggle almost exactly the same amount as their twins.

The difference was so small (less than 1 standard deviation, or $\sigma$) that it could easily be just random luck. When they looked at the specific group of stars found by the TESS space telescope (a very clean, modern sample), the result was the same: no difference in the jiggle. Even when they looked at the "Ultra-Hot Jupiters"—the ones orbiting so close they should be destroyed the fastest (periods less than 2 days)—they still didn't find a younger population.

The Warm vs. Cold Confusion

The paper also tackled a side mystery. Other scientists had noticed that stars with "Warm" or "Cold" Jupiters (planets that orbit further away) seemed older than Hot Jupiter stars. The old theory was: "Hot Jupiters are young because they get destroyed; Warm Jupiters are old because they survive."

The authors found a different explanation. The Warm and Cold Jupiters were mostly found by the "radial velocity" method, which prefers looking at quiet, slow-rotating stars. These stars happen to be older naturally. The Hot Jupiters were found by "transit" methods, which look at different kinds of stars. So, the age difference wasn't because Hot Jupiters were dying; it was because the two groups of stars were selected in different ways. It's like comparing the age of professional marathon runners (who are fit and young) to office workers (who might be older); the difference isn't because the runners are special, it's because you picked two different groups of people to begin with.

The Bottom Line

So, what does this mean for the fate of Hot Jupiters? The paper suggests that we cannot use the "jiggle" of stars to prove that a large number of Hot Jupiters are being swallowed up by their stars while the stars are still young. The evidence for this "tidal destruction" is not as strong as we hoped.

The authors are careful to say they haven't proved that Hot Jupiters never get destroyed. They just say that the specific method of looking at star movements doesn't show a big pile of young, planet-less stars. It's possible that the planets are dying, but the process is slower than we thought, or that the stars we are looking at are just too few to see the pattern.

In the end, the universe is still full of mysteries. Hot Jupiters might still be getting eaten by their stars, but if they are, they aren't leaving behind a trail of "young" stars that we can easily spot with our current speedometers. The authors suggest that we need to be very careful when using star movements to guess the age of planetary systems, especially when the differences are small. The story of the Hot Jupiters is still being written, but this chapter tells us that the "young star" clue might have been a red herring all along.