How many additional exoplanets are in known systems?

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

But even within known planetary systems, there could be planets we’ve overlooked. Perhaps their orbit doesn’t pass in front of the star from our vantage point, or the evidence of their presence is buried in data noise. How might we find them? A recent paper on the arXiv preprint server offers an interesting approach.

Rather than combing through the observational data trying to extract more planets from the noise, the authors suggest that we look at the orbital dynamics of known systems to see if planets might be possible between the planets we know.

Established systems are millions or billions of years old, so their planetary orbits must be stable on those timescales. If the planets of a system are “closely packed,” then adding new planets to the mix would cause the system to go all haywire. If the system is “loosely packed,” then we could add hypothetical planets between the others, and the system would still be dynamically stable.

To show how this would work, the authors consider seven planetary systems discovered by the Transiting Exoplanet Survey Satellite (TESS) known to have two planets. Since it isn’t likely that a system has only two planets, there is a good chance those systems have others. The team then ran thousands of simulations of these systems with hypothetical planets, calculating whether they could remain stable over millions of years.

They found that for two of the systems, extra planets (other than planets much more distant than the known ones) could be ruled out on dynamical grounds. Extra planets would almost certainly destabilize the systems. But five of the systems could remain stable with more planets. That doesn’t mean those systems have more planets, only that they could.

One of the things this work shows is that most of the currently known exoplanetary systems likely have yet-undiscovered worlds. This approach could also help us sort systems to determine which ones might deserve a further look. We are still in the early stages of discovery, and we are gathering data with incredible speed. We need tools like this so we aren’t overwhelmed by piles of new data.