The Search for the Inbetweeners: How packed are TESS planetary systems?

TL;DR

This study analyzes seven TESS planetary systems to determine the potential for additional unseen planets, revealing that most systems have room for more planets, except for TOI 421 which is dynamically constrained.

Contribution

It provides a detailed dynamical analysis of seven TESS systems to assess the likelihood of undiscovered planets existing between known planets.

Findings

Five systems have ample room for additional planets.

TOI-1670's known planets prevent additional planets due to perturbations.

Adding Earth-mass planets to TOI 421 often causes instability.

Abstract

In this work, we examine seven systems discovered by TESS, to see whether there is any room in those systems for an additional planet (or several) to lurk unseen between the two planets already confirmed therein. In five of those systems (namely HD 15337; HD 21749; HD 63433; HD 73583 and LTT 3780) we find that there is ample room for an undiscovered planet to move between those that have already been discovered. In other words, as they currently stand, those systems are not tightly packed. In stark contrast, the perturbative influence of the two known TOI-1670 planets is such that additional planets in between are ruled out. The final system, TOI 421, is more challenging. In the vast majority of cases, adding an Earth-mass planet to that system between the orbits of the known planets caused catastrophic instability. Just ~1.1% of our simulations of the modified system proved dynamically stable on a timescale of one million years. As a result, it seems that there is very little room between the two known planets in the TOI 421 system for an addition unseen world to exist, but the existence of such a planet can not be definitely ruled out on dynamical grounds alone.