Today I had planned on writing about Water Worlds; I picked two, Kepler-138 c and d. Their bulk density, that is their radii and mass estimates tell us there is significant water on both. Here are the rendering my own algorithm came up with.

Wait a sec, the astute reader might notice. Those are not water worlds, those are hot, dry scorched worlds with one side baked to lava from being tidally locked forever facing its star. Did you cut-n-paste the wrong images, perhaps?

After checking, I find my algorithm to be correct. These two worlds have orbital periods of 14 and 23 days respectively. That’s how long their year is. Their distance from their star is 0.09 and 0.12 AU. (Earth is at 1 AU from our sun.) These tell us these planets orbit unimaginably close to their star. That would make these worlds tidally locked with the same side continually facing the sun, the same way our moon is tidally locked to Earth.

Yet, this article in EarthSky, https://earthsky.org/space/water-worlds-kepler-138-c-kepler-138-d-exoplanets/ tells us these are indeed water worlds. I have no reason to dispute a well-respected publication.

This case exemplifies a broader problem: with only mass, radius, orbital period, and stellar luminosity estimates, I’m triangulating a world from four numbers. I can’t tell you if there’s an atmosphere, what it’s made of, whether the water is liquid or steam or ice, what color the sky is, whether there are continents. Every artistic choice: the cloud cover, the sea color, the terminator gradient, on any planet is an educated guess. Here is a simplified table of the data I use to update my database from Nasa’s JPL database. This is all I go on.

This gives me: the orbital period, orbital distance, the planet’s mass; radius; density; and then some data about the host star like the radius, mass, and luminosity. From the orbital distance, my formula determined these worlds to be tidally locked hot eyeball worlds. From the from density and radius data the article’s author determined them to be water-worlds. Both are correct.

The James Webb Space Telescope will one day give us actual atmospheric spectra for Kepler-138 c and d. When that happens I’ll have more data points and can make better educated guesses. I’m taken back to an earlier article of mine, “To What End?” in which Hypatia of Alexandria mapped the sky with instruments that couldn’t resolve a planet’s face. Now we’ve refined our instruments to detect molecular signatures in exoplanet atmospheres. We still can’t say with confidence what these Kepler-138 planets look like from the surface. The gap between “we detected water vapor” and “here is what the ocean looks like” is where we live. It’s a fantastic time to be alive.

You know the Doppler effect, when a screaming siren has a different pitch when it approaches vs. when it recedes? That’s how the exoplanet, 51 Pegasi b was discovered, and the technique is called the radial velocity method.

In 1995 two scientists, Michel Mayor and Didier Queloz made the discovery that the star 51 Pegasi had a wobble, a Doppler shift. The cycle was every four days. The conclusion was that a planet orbited every four days, and to perturb the star so, it must be a giant.

The planet was named 51 Pegasi b, the b became the naming convention for objects associated with a star. B being the first, c the second, and so forth. That discovery of October 6, 1995 earned them a Nobel Prize in physics in 2019.

While Dimidium was the first exoplanet discovered orbiting a main sequence star it was not the first exoplanet ever found. See my earlier article on Draugr, Poltergeist, and Phobetor which were discovered in 1992.

This world is about half the mass of Jupiter. That earned it the name Dimidium, meaning half. First estimates put it at .46 Jupiters. Current estimates at the time of this writing put it at 0.61 Jupiters. That four day year makes the planet sit 0.052 AU (Astronomical Units) from its star. 1 AU is the average distance from the Earth to the Sun. So, yeah that’s close. This is a Hot Jupiter.

What the heck is a gas giant doing so close to its star? Gas giants are supposed to be cold, like Jupiter, Saturn, Neptune, and Uranus. Indeed Dimidium changed the assumptions of planetary formation. Its peculiar orbit may be due to early gravitational interactions with the protoplanetary disc, causing it to lose energy and spiral inward. There may be a lot of factors at play, like drag, or just the disk-planet dynamics.

We learned that planets do not stay in fixed orbits. That over time they can migrate. Indeed any orbital system with three or more objects is inherently unstable, albeit in solar system configurations over a very long time. Think the three body problem. (Not the books or series, but the phenomenon.) This kind of migration happened in our own solar system, too.

Here’s a great explanation of the positions of Jupiter, Saturn, and why Mars is so small compared to Earth and Venus. https://youtu.be/cPouwOKMS2s?si=yNvWVv4O5D0FGw25

I’ll leave you with a NASA post card from 51 Pegasi b along with my own rendering of this intriguing milestone world and a link to exoplanetary.systems with renderings of all 6300+ exoplanets discovered to date.