Tenebrous
BigG > Mad Celestial Host
I think Thanos has a chance, but he has limited options.
Barring any plot device object, Thanos' best chance is to use Galactus' tech against him somehow, since Thanos has stated previously that the workings of Galactus' tech is beyond his understanding (Thanos limited series) and since Thanos used Galactus' own tech in annihilation because only by using galactus' own tech could thanos achieve his objective (transforming G into the power syphon/source for annihilus).
alternatively he can figure out a way to cloak the planet, which has been used successfully before to avoid detection by G.
lastly, he can rig some kind of interplanetary explosion like in thanos #6 and teleport just before impact. the planet is gone of course, but thanos prevented G from devouring it
In any case, Thanos actually did have to deter G before from eating Rigel...Thanos was unsuccessful, but he did have much less than a week to prepare.