Or, a record for the biggest thing lost! Or, to loose your car keys is careless, but to lose a planet??
As some readers will know, I have my own theory of how planets form from the dust of the accretion disk around stars. The dust is actually micron sized, and finer than an average smoke distribution, and as the gas falls into the star, it gets hot, its potential energy lost being converted to heat, and when it gets hot enough it glows. Of course, it also glows by scattering light emitted by the forming star hence we observe those disks. Standard theory assumes planets form through gravity. The problem here is gravity is far too weak to aggregate this dust, so the standard theory assumes that somehow it forms planetesimals and there is an even distribution of these out to about 32 A.U. (one A.U. is the Earth-Sun distance) where their density falls off dramatically for some unknown reason, although it is necessary they do to prevent more planets being formed further than Neptune. While this is needed to make the theory fit the facts, it is not exactly enlightening, and it is far from clear why Neptune has far more mass than Uranus.
Anyway, I find all this unsatisfactory on many counts. My concept is that getting started involved chemistry, or physical chemistry, in which case what you get depends on the local temperature during accretion. In principle this is quite predictive; in practice not so much because we do not know what the temperatures were for a given system because all evidence of the disk for all systems older than 40 My is long gone. The temperatures depend on the potential energy converted, but that depends on how fast the star is accreting, i.e. the rate of conversion, and for a given size of star, this can vary by a factor of ten. However, there are proportional predictions, i.e. if you have a distribution of planets, it predicts how they should be spaced and what their relative properties should be. Thus besides explaining why Neptune is more massive than Uranus, and there are no planets further out (actually, it does predict the possibility of more, but very far out). It also predicts no life under ice at Europa because the theory predicts there are no significant amounts of nitrogen or carbon, which is verified by observation.
When I published my ebook on planetary formation in 2012, by and large the known observed data matched predicted data surprisingly well, although there was not that much respectable data on multiple systems and for single exoplanets, variation up to an order of magnitude was “tolerable”. However, much of what was generally predicted appeared right, thus red dwarfs, because they had much less material, were expected to have planets closer to the star, and they were. But there was one annoying feature. The star Fomalhaut has a rather odd accretion disk around it, and it appears with the brightest part as a ring very distant from the star. My guess is the star is in the process of ejecting the disk, but maybe that is wrong. Anyway, the disk had a planet that could be seen in a telescope. The problem was, it was somewhere between 100 – 250 A.U. from the star. That by itself was no problem; the star is huge, so planets will be well spread. The problem was, if it were that bright, you should be able to see others, and there were no others. As it happens, this week the problem has gone away because the planet has gone away. More specifically, it was never there. Maybe that should have been suspected. Its light distribution was quite odd, being surprisingly bright in the visible, but seemingly less so in the infrared, which was indicative of surprising heat. Newly formed gas giants can be very hot, due to the energy of the gas falling onto them, so nobody took much notice. However, after a very short time it faded, and now it cannot be seen. What we now think happened was that a collision took place between two objects about 250 km long, and we saw the extreme heat so generated. A very rare occurrence indeed.