Down in flames

The more exoplanets we find, the more they challenge our notion of how planets should be.

For the many of us who grew up watching Star Trek and Lost in Space and Doctor Who, it's understandable that we picture planets around other stars as being pretty much like the ones we have here in our own Solar System -- either small and rocky like the Earth, or gas giants like Jupiter and Saturn.

The truth is, there is a far greater variety of exoplanets than we ever could have dreamed of, and every new one we find holds some sort of surprise.  Some of the odder ones are:

• TrES-2b, which holds the record as the least-reflective planet yet discovered.  It's darker than a charcoal briquet.  This led some people to conclude that it's made of dark matter, something I dealt with here at Skeptophilia a while back.  (tl:dr -- it's not.)
• CoRoT-7b, one of the hottest exoplanets known.  Its composition and size are thought to be fairly Earth-like, but it orbits its star so closely that it has a twenty-day orbital period and surface temperatures around 3000 C.  This means that it is likely to be completely liquid, and experience rain made of molten iron and magnesium.
• 55 Cancri e, nicknamed the "diamond planet."  Another "hot super-Earth," this one is thought to be carbon rich, and that because of the heat and pressure, much of the carbon could be in the form of diamonds.  (Don't tell Dr. Smith.)
• PSR J1719−1438, a planet orbiting a pulsar (the collapsed, rapidly rotating core of a giant star).  It has one of the fastest rates of revolution of any orbiting object known, circling its host star in only 2.17 hours.
• V1400 Centauri, a planet with rings that are two hundred times wider than the rings of Saturn.  In fact, they dwarf the planet itself -- the whole thing looks a bit like a pea in the middle of a dinner plate.

We now have a new one to add to the list -- BD+05 4868 Ab, in the constellation of Pegasus.  Only 140 light years away, this exoplanet is orbiting so close to its parent star -- twenty times closer than Mercury is to the Sun -- that its year is only 30.5 hours long.  This proximity roasts the surface, melting and then vaporizing the rock it's made of.  That material is then blasted off the surface by the stellar wind.

So BD+05 4868 Ab is literally evaporating, and leaving a long, comet-like tail in its wake.

The estimate is that each time it orbits, it loses a Mount Everest's worth of rock from its surface.  It's not a large world already, and the researchers say it is on track to disintegrate completely in under two million years.

"The extent of the tail is gargantuan, stretching up to nine million kilometers long, or roughly half of the planet's entire orbit," said Marc Hon of MIT, who co-authored a paper on the planet, which appeared this week in Astrophysical Journal Letters.  "The shape of the transit is typical of a comet with a long tail,.  Except that it's unlikely that this tail contains volatile gases and ice as expected from a real comet -- these would not survive long at such close proximity to the host star.  Mineral grains evaporated from the planetary surface, however, can linger long enough to present such a distinctive tail."

[Image licensed under the Creative Commons Marc Hon et al. 2025, submitted to AAS Journals, BD+05 4868Ab simulation dust cloud (Figure 12), CC BY 4.0]

So we have a new one to add to the weird exoplanet list -- a comet-like planet in the process of going down in flames.  Not a place you'd want to beam your away team to, but fascinating anyhow.

Makes me wonder what the next bizarre find is going to be.  The universe is like that, isn't it?  We think we have it figured out, then it turns around and astonishes us.

I, for one, think that is fantastic.

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