Planetars and Solars are angelic beings in SLASH'EM which are both essentially stronger versions of Archons. They possess the same speed, alignment, armor class, magic resistance and spellcasting abilities of an Archon. They merely have a higher base level, and their attacks do more damage: a Planetar does twice as much, and a Solar approximately 250% as much.
A Planetar is truly an awesome fighting force, dealing large amounts of physical damage through its weapon attacks, and possibly even larger amounts through its psi bolt monster spell. It has a higher base level for casting than an arch-lich.
A Solar has the highest base level of any randomly generated SLASH'EM monster, a distinction it lives up to with its immensely damaging physical and spell attacks.
Before getting the Amulet, the only ways that these monsters can be found are either as minions for lawful characters, obtained through prayer or sacrifice, or possibly in Vlad's Tower or Frankenstein's lab. Their difficulty level is simply too high to be seen earlier; a level 30 character, reaching the castle at level 45, is still not going to see even a Planetar there.
As with the devas, Planetars and Solars constitute a major reason that lawful is an easier alignment in SLASH'EM: not only are they peaceful to them, but can be (and often are) given to them as minions. As they cannot be met until one has obtained the Amulet or is reasonably near to doing so, there is no reason to fight either monster, even if one encounters them. By this time your goal is to reach the Elemental Planes anyway. A player in speed boots will be slightly faster than them, and wands of teleportation can work wonders to make them go away. Thus, just as one should keep a wand of digging handy for a chance encounter with a giant shoggoth, one should keep a wand of teleportation handy for these monsters. They are not covetous and will not teleport to meet you.
If you insist on fighting these, either hope that your character is truly maxed out in fighting capabilities (+7 artifact weapon, expert weapon skill, AC around -45), or have a cockatrice corpse handy. Note that they are guaranteed a shield of reflection, and resist just about every type of damage. Using the disarm technique or a bullwhip to rob them of their weapon is a good way to reduce the damage they do, as well as bag yourself their guaranteed artifact weapon. They are also not immune to drain life, so the un-reflectable wand of draining can also be useful against them. Even with this, though, they remain truly formidable opponents.
Conversely, they also make truly formidable pets, and the "blessed figurine of an Archon" that is a standard pet wish in vanilla NetHack is supplanted here by a figurine of a Solar. Pet Solars will quickly raise their levels to 49, will not go feral, and (even when generated from a figurine) are guaranteed Sunsword or Demonbane if these have not already been generated. (This is in effect a shortcut to a guaranteed artifact wish, without breaking artifact-wishless conduct.) They will not wear body armor, but will wear other armor, although they are so powerful it is rarely necessary to give them the boost.