A long time ago, I studied comets during my years in graduate school. We studied cometary plasma tails, and found a lot of interesting things. But, we didn’t know at the time that a comet can have an aurora. That all changed with the in-depth study of Comet 67P/Churyumov-Gerasimenko. The Rosetta mission studied that comet in great detail, including emissions from and around the nucleus. Among those emissions: far-ultraviolet light that is generated when electrically charged particles from the Sun interact with the gassy coma surrounding a cometary nucleus.

From Dayglow to Cometary Aurorae

Initially, scientists interpreted the far-UV emissions as part of the “dayglow” surrounding the comet. When they re-analyzed the data, they found the emissions. The solar wind remains the culprit, just as it plays a role in forming the plasma tail of a comet. Charged particles in the solar wind interact with the gases in the coma. That actually causes water and other molecules to break apart and the resulting atoms give off far-ultraviolet light. And, that’s what forms the aurora around the comet.

Implications of Cometary Aurorae

Of course, the formation of such emissions at a comet tells scientists something about the solar wind, its particle loads and intensities. Studying what happens at a comet can give better insight into changes in the solar wind over time. It’s particularly important for understanding space weather, which is caused by the solar wind and its interactions with planetary magnetospheres. Space weather is a natural phenomenon and can affect satellites and astronauts in orbit around Earth. It would definitely have an effect on missions beyond Earth, too.

Back when we were studying comets and their plasma tails, we depended on a solar-orbiting spacecraft called Ulysses. It gave us information about the solar wind as it left the Sun. That allowed us to follow the effects of the solar wind on the comets we were studying. In particular, it helps shape the plasma tail, and disturbances in the solar wind definitely showed up at the comets some hours or days later.

New Data, New Ways of Seeing Comets

The Rosetta spacecraft is now “one” with the comet it studied. Its data adds a whole new dimension to what we know about comets and their interactions with the solar wind. Up until this discovery, I am not sure people thought about cometary aurorae. But, then again, back in the 1990s, we never thought comets could have x-ray emissions. Those were discovered by the Rœntgen X-ray Satellite and Rossi X-ray Timing Explorer at comet C/Hyakutake 1996 B2 and are also thought to be due to an interaction between the comet and the solar magnetic field entrained in the solar wind. This recent discovery is another in a long line of unexpected surprises about comets.

Ever since the first extrasolar planets were found in the mid-1990s, we’ve been fascinated by what they could be like. Of course, we hope to find rocky worlds. And, astronomers have found gas- and ice-giant worlds. But, how about diamond planets?

As a science fiction reader, I’ve “visited” many strange planets throughout the years. Heinlein wrote a story about Venus as a sloppy, hot, wet hellhole. He had people living and working there. The astronomer Carl Sagan and others thought that Jupiter’s clouds could possibly host strange life forms suited to the conditions there.

Weird Planets in Science Fiction

I remember reading with great fascination in Arthur C. Clarke’s Childhood’s End about one character’s exploration of worlds with high gravity, where aliens flitted about with wings. He also described another world that hosted a weird bit of life in the shape of a giant eye. It was pretty clear that the planets shaped the life forms.

That makes sense, once you understand how stars and planets form. Their birthplaces are giant clouds of dust and gas. The resulting objects orbiting the star are going to have the same “bulk composition” as their birth cloud. If the cloud is like the one our solar system was born in, it has a much lower ratio of carbon to oxygen. Plus, as we know from Earth, it had more silicates. That is, in fact, the general composition of Earth. It does have carbon—in smaller abundance.

Could There be Carbon Planets?

A group of scientists at Arizona State University and the University of Chicago took a look at the types of known planets. Their goal was to figure out if a carbon-rich planet could exist. If so, that might imply the existence of diamond planets.

Thanks to the Kepler and Tess missions, along with Hubble Space Telescope, and a slew of ground-based observations, we know that, as of today, (according to the NASA Exoplanet Archive) 4,276 of these discoveries are confirmed as exoplanets. There are thousands more candidates that have to be confirmed. The hunt for planets is a slow and painstaking process, and I’m sure astronomers will be finding and confirming more.

The current missions and observations are giving astronomers a pretty good look at what the known planets are made of. They also get a good idea of what they might look like, plus some clues about whether or not they could be habitable. What if the proto-stellar nebula has more carbon in it? Then, you might expect to see carbon-rich planets.

Making Carbon/Diamond Planets in the Lab

The research teams devised an experiment to simulate how such planets might form. They mimicked the interior of carbide exoplanets using high heat and high pressure in the lab, using high-pressure diamond-anvil cells. First, they immersed silicon carbide in water and compressed the sample between diamonds to a very high pressure. Then, to monitor the reaction between silicon carbide and water, they conducted laser heating at the Argonne National Laboratory in Illinois. They took x-ray measurements while the laser heated the sample at high pressures. The result? With high heat and pressure, the silicon carbide reacted with water and turned into diamonds and silica.

So, if conditions are right (meaning ample amounts of water, silicates, and carbon), it’s possible that the universe could be creating carbon-rich planets made of diamonds and silica.

Life on a Diamond Planet?

So, what if a planet is made of diamonds? Could it support life? That’s an excellent question, and so far, nobody has found life of any kind on other worlds. But, life is tenacious, and it doesn’t have to live on a planet just like Earth. However, a carbon planet might be stretching it a bit. Having a planet made mostly of compressed carbon, a diamond planet, can pose new problems for life.

Planetary scientists and astrobiologists are hoping to find planets with the right properties. They also have to be at the right location around their stars so life could exist. So far, however, nada. And, it gets worse for carbon-rich planets. They likely do not have the properties needed for life. Why would that be?

A Diamond Planet Compared to Earth

Earth life grew up in conditions unique to Earth. Our planet is geologically active (an indicator of habitability), for one thing. If the results of this study work for actual carbon-rich planets, then they are too hard to be geologically active. Without geologic activity, it’s tough to build up an atmosphere to support life. The planets may not meet all the requirements that living creatures need. It doesn’t necessarily mean that there isn’t a carbon-rich planet out there teeming with some kind of life. However, the odds are stacked against it.

Still, imagine a diamond planet! I remember goggling at the idea of a diamond core in Jupiter. I read about THAT in one of Clarke’s books). So, it’s not so far-fetched to think about a diamond world somewhere out there. It could be orbiting a distant star, along with its rocky and gas/ice giant siblings.