This happened during the third era of the known Universe, before the Milky Way merged with another galaxy and became the Milky Cluster.
A group of exiles from a long-lost culture, who called themselves "The Runaway Pets," came upon the bloated red husk of a dead star. They made an orbit around this star. And while their slow ship fell in a curve through the star system, the passengers came out of the stasis-tubes to gaze upon the red sphere of gas.
The Runaway Pets, carbon-based bipeds with multispectrum vision and an oxygen metabolism, were compelled to leave their mark on this apparently dead star system - just as their genetic ancestors had once drawn up territory by leaving urine tracks.
Literally, the Runaway Pets became obsessed with the idea of finding a planet and peeing on it. (They are not proud of this compulsion.)
It did not take them long to find three remaining solid planets that circled the red star. The Pets sent out probes to scan the planets for eventual dangers. The probes sent back signs of activity beneath the surface of each planet. And soon after, the planets sent messages to the Pets.
The messages came in many forms: radio, laser and t-port. The language was easily decoded as an ancient galactic dialect, and a dialogue was established, presentations made, platitudes exchanged. This was not the first time the Pets had talked to another culture. But it was the first time a culture had presented itself to them as a whole planet.
The planet closest to the dead star called itself "Mars." It was quite old, and cranky. Further out in orbit was a much larger planet which went by the name "Jupiter." Jupiter had the jovial manner of a corpulent being. And further out still, a large planet orbited the star. It refused to give away its name to the travelers, but Jupiter told them that the third planet was named "Neptune." Neptune replied that Jupiter had no right to tell strangers his name.
Mars told them both to behave.
The Pets asked them for permission to land, and to "make their mark" on the surface.
Jupiter warned them that even though he was not as large as he once had been, his gravity was still dangerously strong for bipeds, but "little Mars" would offer them no resistance. Mars grumbled that Jupiter was an "asshole." This puzzled the Pets greatly; why would a planet-shaped being refer to the anatomy of wholly different life forms?
"We have an idea where you came from," the Pets told the three planets by laser transmission. "You must have evolved from the dominant life forms of this star system. But why are you still here? Your star is dead."
"None of your business," replied Mars.
"We like it here," said Jupiter.
"We are trapped," said Neptune. "Prisoners of..."
Then there was a strange silence from the planets.
The Pets repeated their request to land, but they received no further answers for a while, and it seemed the planets were communicating among themselves.
Finally, Mars replied: "Land here. But don't throw any garbage on me."
And there was much rejoicing among the Pets. It had been a long time since they last "made their mark" on a planet.
They left their mothership in orbit around Mars, and flew a group of smaller scout ships down toward the ancient surface. In the coppery, dim light of the dead star, Mars resembled a crimson drop of coagulated blood — smoothed out and devoid of mountains, but wide and small cracks ran across its entire surface. It was obvious that the surface had once melted when the star underwent its death throes and swelled to enormous size.
There was no welcoming committee; the Martian intelligence communicated through antennae, but stayed beneath the surface.
With blissfull relief, the hundred Pets scattered out across the planet's surface and peed on it.
One of the Pets made to toss away a piece of junk on the surface. But then a kilometer-long mechanical arm shot up from a crack in the ground, bent into a snake-like undulating shape, and reached along the ground for him, snapping a hundred metal claws and hands. The Pet shrieked and retreated to his ship. The Pets readied the ship for immediate take-off, but changed their minds when the giant arm slid back into the ground, and bothered them no more. The Pets gathered in a pack, and admired the eerie landscape and the awesome red star which covered half the sky.
"She was beautiful in her youth," Mars told them. "I could spend years just admiring her corona."
One of the Pets asked the planets if all three of them had been born at the same time.
Mars told them that he was the oldest surviving one. His comment must have unsettled Jupiter and Neptune, for they sent out a burst of incoherent signals which the Pets could not decipher.
Then, several Pets asked the obvious question: "Why did you say you were the oldest surviving planet? How many others were there?"
All three planets replied with silence.
Now the Pets' curiosity had been baited, and they performed a deep radar scan of the planets. This proved harder than they had expected: the planets put up energy barriers to shield themselves from the visiting voyeurs.
But the Pets were a persistent breed. They tricked and feinted and bluffed, and got their snapshots of the planets' insides. The three-dimensional images of Mars, Jupiter and Neptune showed just who "they" were: sentient metal and silicate structures, many kilometers thick, hidden deep beneath the rock surface. Gravity-converters in the planetary cores produced thermal energy to sustain and renew the sentient superstructures. Their sun had long since ceased to warm them.