Hi everyone,
I’ve been reading about exoplanets and noticed that some of them orbit their stars in just a few days—or even hours! How is it possible for a planet to orbit so close without getting torn apart by tidal forces or burning up from the heat? Are these planets stable long-term, or are they eventually destroyed? Would love a scientific explanation!
Thanks!
Comments
We know how close a planet has to be to be torn apart by tidal forces, depending on the density of the star and planet, it can be just a few radii away from the star without becoming a ring. A bloated star can’t rip apart a rocky planet at all until they collide. You can read more about the Roche Limit
I believe a less massive star would have shorter orbits outside its Roche Limit.
These super short orbital periods maybe aren’t common but the way we find exoplanets means we’re biased to find these dizzy planets first because we detect them when they transit (pass in front of) their star.
To add if you are using the stars spectrum to look for a “wabble” as the planet tugs on the star you need to watch for a number of orbits in order to get enough signal. Larger orbits take more time so it’s easier to detect closer planets
There’s a bit of selection bias going on here.
There are a few popular ways we detect exoplanets. In a nutshell, it’s these two methods:
Transit Photometry
We can point a telescope at a star and keep track of the brightness vs. time. This creates a jittery brightness plot, but any planet that transits between the star and the telescope will dim the star very slightly from the perspective of our telescope.
Because the signal is noisy, you need to look for periodic dimming of the star. This requires a fourier transformation of the data, where the random noise falls out of the data and you’d be able to clearly see the magnitude of the dimming, and at which frequency that dimming happens. Doing this method requires several transit events from the same source. This heavily favors both large planets (larger dimming effect, more noticeable) and planets closer to the star (shorter orbital period; means more transits between star and telescope).
There just simply hasn’t been enough time for planets with long orbital periods to be detected with this method. We’ve been pointing telescopes at the sky for maybe 60-70 years. Neptune’s orbital period is 165 years. We literally can’t have detected Neptune via this method
Radial Velocity
Transit Photometry requires the planetary plane of that solar system to line up with our telescope. If that’s not the case, we can try looking at the wobble of the star vs time. Due to tidal and gravitational forces of planets orbiting the star literally pulling on the star. It’s slight shifts in position and brightness can inform us on short orbital, high-mass planets.
Conclusion
Our detection methods highly favor planets with high masses, large diameters, and close proximity to their host star. There are certainly more, longer period planets out there; they’re just a pain in the butt to detect.
it’s probably detection bias. It’s easier to spot larger planets close to their stars. Much harder to spot the wobble of a planet that has a long orbit. As an example it takes 11.86 years for Jupiter to make one orbit. Do you think we would be able to spot that with the various detection methods?
It has to do with relative sizes. Some stars are only 0.1% the mass of the sun. That makes the tidal stress much lower (in those cases). Those stars are also much more significantly impacted by the gravity of the planet as their masses are closer so we have a detection bias in those instances.