I recently came across an article titled “Astronomers Find a Waterworld Planet With Deep Oceans in the Habitable Zone“. Curious what they actually found, I clicked through to the article. It was about what I expected.
The entire subject of discovering exoplanets is one that does not fill me with confidence. I get the basic approach used, which is looking for regular dimming of stars caused by the transit of a planet in front of the star as it orbits the star. And, indeed, you would expect a planet orbiting a star to (slightly) dim the light coming from that star if you’re lucky enough for the planet to pass right in front of it relative to us. That said, when I say slight, I mean slight. To put it into perspective, our sun has a diameter 109 times larger than the diameter of the earth. In terms of cross-sectional area, that means that the earth’s shadow is about 1/10,000th that of the sun’s. It will block out a little more of the sun than that, since it’s a few million miles in front of the sun rather than directly in front of it, but since we’re observing stars that are light-years away, it won’t be that much more. Jupiter, which is nearly as large as planets can get (as a gas giant’s mass goes up much past Jupiter’s, its gravity causes it to contract), would block out about 1/100th of the sun. So what astronomers are looking for is somewhere between a 1% dimming and a 0.01% dimming.
Even less confidence inspiring, when you look into the actual data, the stars in question are generally around 1pixel big in the images that they’re using. This isn’t always the case, of course, but the stars are never more than a few pixels. In the article in question, when the researchers turned to a much higher resolution telescope, they were able to distinguish the two stars of the binary system where the “waterworld” orbits the larger of the two within the habitable zone. (If you’re not familiar, the habitable zone of a star is the distance away where the heat from the star would result in liquid surface water, as we have here on earth. Too close and the planet will be too hot and the oceans will boil off, too far and they will freeze.) Oh, and these two stars are orbiting each other from roughly twice the distance that Pluto is from the sun. And the high resolution telescope was able to make them out as two distinct source of light.
No one has ever seen this supposed “Water world”. What we have is a periodic dimming of the host star. From the magnitude of that dimming we can calculate the size of the thing crossing in front of it. From the period of the dimming and the time between the dimming we can calculate the orbital period and thus the distance from the star. From the size and orbital period we can calculate the mass, and hence the density.
That last part is the basis of the claim for a “water world” came from, by the way. The density of the planet that was detected is too low to be a rocky planet like earth, and too high to be a gaseous planet. Since it’s in the habitable zone of its star, it’s unlikely to be icy, and so it is a good candidate for being a water world. This in no way justifies calling it a water world, nor does it justify the artist’s rendition of what the surface of it might look like that’s in the article (which is just a picture of the sun setting over the ocean here on earth). It also doesn’t justify the Star Trek like artist’s rendition of the planet near to a sun-like star. The star that the planet is orbiting is a red dwarf. They’re called red dwarves because they don’t put out white light like our sun does. If you look up TOI-1452A (the red dwarf star; TOI-1452 b is the planet) it has a surface temperature of 3185k. It’s not that it puts out literally no blue light, but it puts out very little. This is the dingy yellow-orange light of a low wattage “warm white” incandescent bulb. Oh, and the star only puts out 0.7% of the light that our sun does.
These sort of articles really annoy me because they pretend to have an enormous amount of certainty that we don’t have. What’s actually going on is a little bit of data and a whole lot of calculations. This is interesting, but it does a great disservice to people to pretend that what we have is a lot of data. We don’t.
Moreover, these are all unverified calculations. No one alive today is ever going to set eyes on a photograph of one of these planets to get an independent source of data about their size or composition, or even their existence. It took nine years for the New Horizons probe to fly out to Pluto. Here’s the best picture Wikipedia has of Haumea, a dwarf planet in our solar system:
Haumea is only about 10 AU further away from the sun that Pluto is. (An AU is the distance from the earth to the sun.) Here’s Eris, which is more massive than Pluto, though not quite as large, and which is much further away:
Eris is, at its farthest, about twice as far away from the sun as Pluto. And this is the best picture that we have of it. (Or at least it’s the best picture that Wikipedia has.)
If this is the best that we have of dwarf planets in our own solar system, it suggests that a bit of humility is warranted when it comes to conclusions about planets orbiting other stars. Our galaxy is a big place. There’s no reason to suppose that there is nothing besides exoplanets which will regularly result in the slight dimming of a star’s light. That’s not to say that there’s something wrong with going with what we know—that is, with saying that if the slight regular dimming is caused by an exoplanet, then the exoplanet would have such and such properties. If people are going to get tired and drop the “if”, then perhaps it would be better to stop talking about the subject at all.