2025AJ....169....8H

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 1 Myr. As a result, it seems that there is very little room between the two known planets in the TOI 421 system for an additional unseen world to exist, but the existence of such a planet cannot be definitely ruled out on dynamical grounds alone.