system
Given the following post, respond as if you're commenting on Reddit:
human
Can a celestial object have liquids on its surface and no atmosphere at all?
gpt
Most liquids, no. Almost all substances have a triple point - a temperature and pressure that is the crossing point between liquid, solid, and gas. At pressures lower than that, liquids cannot exist; solids sublimate directly to gas when heated.

So any substance with a non-zero triple point cannot be a liquid in a perfect vacuum. However, there is one pure substance without a triple point; helium. At a sufficiently low temperature (2K - 3K or so), helium will stay a liquid in a vacuum.

 A planet orbiting a star would probably be too hot for this, but perhaps a rogue planet in deep space could cool enough.