There are no radio signals. Radio doesn't work very well underwater.

They probably don't have subspace communications but they check.

No subspace either!

The ship takes a lot of data and circles the planet for a local day, mapping the light density.

Then it leaves.

Deep underwater, in the shining cities, the handful of mermaids fascinated enough by the night sky to make a study of astronomy despite the difficulties posed by the intervening surface of the water are going bananas over the latest readings from their instruments.

After three days, several ships approach the planet, and one slowly comes in for a landing.

A few hours later, the shore the ship is resting on is approached by half-a-dozen person-sized figures.

From the waist up, they're shaped mostly like Amentans--two arms, ten fingers, a head the right shape and size. From the waist down, powerful tails propel them rapidly through the water. They're covered from head to tail with shimmering scales, and in place of hair, undulous fins stretch from the tops of their heads to the smalls of their backs. 

Two of them are carrying a box between them. One of them has four babies attached to their torso.

The Amentans have used these hours to determine that the atmosphere is safe to breathe, set up tents and various observational equipment, and start assembling their sea probe, but they haven't yet released the sea probe.

When the merpeople approach, the Amentans assume teeth-hiding smiles and open, empty-handed body language.

The mermaids stop several yards from shore and set up camp on a shelf of rock. They open the box and pull out a tent-like thing and several of what resemble space suits. While the others are setting up, the two that were carrying the box don suits and swim the rest of the way to shore. They crawl out of the water and mirror the Amentans' open-handed gesture.

"Hello!" says an Amentan with blue hair (there are also two greys, a couple greens, and a yellow visible; there are several purples in the background).

The mermaid on the left presses two buttons on the front of its suit, and then a crooning like whalesong emits from the suit. 

They've got a recorder going and play it back.

The mermaids are excited about this! They make a combination of crooning and clicking noises at each other, then each one points at themself and emits a set of phonemes that are presumably their names. 

The Amentans play that back too, pointing the speaker at the correct mermaid, and then introduce themselves!

The mermaids are very happy to play the "try to teach each other our languages like one of us is a Feanor" game!

So are the Amentans! Mostly the green ones. They can't pronounce the whalesong but they can get it snipped into bits and chain them together on their computers.

After a while, three of the other mermaids suit up and come out to talk to the Amentans. The last adult hangs out in the shallows, watching the babies as they excitedly interact with the concept of the ocean having a surface. 

The Amentans did not bring any kids, but the merbabies are cute. They take pictures and show them to the merfolk.

The merfolk are charmed by the aliens taking baby pictures! 

One of the merfolk expresses, via a combination of established words and images projected from the suit onto the sand, that if any of the aliens wanted to wade out into the shallows and hold an underwater baby that that would be a great photo op. 

One of the greys is brave enough to do this! Out he wades. Which baby wants to be held?

A silvery baby with blue-green streaks on its sides and an especially fluffy dorsal fin swims up to him, big eyes curious and tiny fingers grasping. 

"Awwww!" says the grey, and he scoops the baby without removing it from the water.

The baby snuggles up, attaching itself to the grey's chest with some kind of suction-cup-like thing on its chest, then pats the grey curiously. It warbles in gleeful interest. 

The other three babies swarm around the grey's legs, curious what these weird twinned tail things are. 

The grey wiggles his toes at the babies and pets the one on his chest gently.

The adult mermaid swims over and smiles and withdraws a handful of soft grey stuff out of a pouch at their waist. The three unattached babies swarm her, and she feeds them, giggling, offering the grey some to feed to the attached baby.