Tuesday, January 2, 2024

How many planets could be in the Kuiper Belt?


Motivated by recent measurements of the free-floating-planet mass function at terrestrial masses, we consider the possibility that the solar system may have captured a terrestrial planet early in its history. We show that ∼1.2 captured free-floating planets with mass strictly greater than that of Mars may exist in the outer solar system, with a median predicted distance of ∼1400 au.

To evaluate the expected mass of the most massive captured free-floating planet as a function of the maximum semimajor axis, given the arguments presented in Section 2, we implement a Monte Carlo simulation.

This is an interesting calculation, but the result of ~1 free-floating planet does not have a reported error and seems extremely sensitive to the mass function. Equation 1 gives the mass function of free-forming planets:

The coefficient on the power-law term has substantial errorbars, changing the result of the mass function by up to 75% if alpha ~ 1. Examining alpha as well, the paper reports that alpha ~ 0.96 \pm 0.47, which would have a substantial effect on the value of the mass function. It wasn't immediately clear how this substantial variation affected the number of planets detected. Figure 1 shows the confidence intervals on the mass estimate for a planet of abundance unity, but I wasn't able to immediately grasp how to convert this into an errorbar on the planet abundance estimate. Then Figure 2 shows the cumulative fraction and expected number of captured free-floating planets, but doesn't show the confidence intervals. So overall, it's hard to assess the free-floating planet abundance estimate from their simulations given the huge spread on the parameters. The estimate changes from ~1 to ~3 with a change of binning, which is expected but also highlights how uncertain the estimate and approach are, so it would been interesting to see a more thorough exploration of this.


A recent study published in the Astrophysical Journal Letters investigates the potential existence of Mars-sized free-floating planets (FFPs)—also known as rogue planets, starless planets, and wandering planets—that could have been captured by our sun's gravity long ago and orbit in the outer solar system approximately 1,400 astronomical units (AU) from the sun.

Scientists currently hypothesize that rogue planets are formed from two scenarios: As part of their own solar system but are then somehow ejected into the cosmos, or they form in isolation. But what is the significance of studying free-floating planets, overall?

After conducting approximately 100,000,000 simulations, the results indicate the potential for the existence of a Mars-sized, or even a Mercury-sized planetary body somewhere in the outer solar system approximately 1,400 AU from the Sun.

Universe Today:

Siraj recommends in his study that future work could include gaining greater understanding of how rogue planets are captured in the first place, along with investigating observational tests to identify where to look in the sky for rogue planets, as well. He also notes how microlensing has become the preferred method in identifying rogue planets based on past studies.

Rogue planet capture is extremely interesting for sure. I'm also super interested in the related phenomenon of JuMBOs (Jupiter-Mass Binary Objects), which are forcing astrophysicists to confront the same tough questions as this study.

Previously on this blog:

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