How to Weigh an Invisible Planet That Hides Half of Itself ✛
A star shivers when a planet orbits nearby: the invisible companion tugs it toward us and away from us. From that wobble, astronomers know a planet is there and can even estimate its weight. But there's a catch. If the planet's orbit is tilted, like a plate seen edge-on, only part of its gravity acts on the star. What we see in the telescope is not the true mass but its shadow.
A single planet can't be weighed this way. But when there are several planets jostling like commuters at rush hour, a simple idea comes into play: overly heavy neighbors will shove each other apart and tear the whole system apart. So if the system is still intact, the planets cannot be too massive. That limit reveals the true weight.
The authors of the new study applied this logic to five known systems. It turned out that around Barnard's star, all planets are lighter than 0.6 Earth masses, making them rocky worlds rather than gas giants. And around HD 184010, three large planets weigh as much as Saturn or Jupiter. Previously, heavier options were allowed, but now they are ruled out: such giants would have long since smashed the system to pieces.
If the orbital inclination were very small, some 'planets' would turn out to be brown dwarfs, objects above the deuterium-burning limit. For instance, for the star HD 215152, Gaia astrometry excludes inclinations below 0.2 degrees, which corresponds to masses greater than several Jupiters.