How many more exoplanets are there in known systems?

One thing we’ve learned in recent decades is that exoplanets are surprisingly common. So far we have confirmed about 6,000 planets, and there is evidence for thousands more. Most of these planets were discovered using the transit method. Although there are other methods. Many stars are known to have multiple planets, such as the TRAPPIST-1 system, which has seven Earth-sized worlds.

However, even within known planetary systems, there may be planets that we have overlooked. Perhaps their orbits don’t pass in front of the star from our perspective, or perhaps the evidence for their existence is buried in data noise. How can I find them? A recent paper on the arXiv preprint server provides an interesting approach.

Rather than combing through observational data in an attempt to extract more planets from the noise, the authors used We propose to investigate orbital mechanics.

Established star systems are millions or billions of years old, so the planets’ orbits must be stable on those timescales. If the planets in a system are “crowded together”, adding a new planet will disrupt the system. If the system is “loosely packed”, hypothetical planets can be added between other planets, and the system is still dynamically stable.

To demonstrate how this works, the authors consider seven planetary systems discovered by the Transiting Exoplanet Survey Satellite (TESS) that are known to have two planets. Since it is unlikely that a star system has only two planets, it is quite possible that there are other planets in those systems. The team then simulated these systems with hypothetical planets thousands of times to calculate whether the planets could remain stable over millions of years.

They found that for two of the systems, extra planets (except those that are much more distant than the known planets) could be ruled out on dynamical grounds. The extra planet would almost certainly destabilize the system. However, five of the systems could remain stable even with more planets. That doesn’t mean there were more planets in those systems, just that they were possible.

One thing this study shows is that most of the currently known exoplanet systems likely have worlds yet to be discovered. This approach is also useful for classifying systems to determine which ones are worth further investigation. We are still in the early stages of discovery and are collecting data at an incredible rate. You need tools like this to avoid being overwhelmed by mountains of new data.