The Doctor, the eponymous star of Doctor Who, doesn’t visit her homeworld very often. This isn’t just because the Time Lords are an irascible bunch; after consulting with several Who-obsessed scientists, it’s clear that Gallifrey is less like the strangely habitable planets of Star Wars and more like the worst place in the universe to live.
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Gallifrey made its first appearance in 1969, but has for the most part remains an off-screen presence. Since the start of the modern era of the show, the Doctor’s homeworld has been seen in flashbacks. It was long thought to have been destroyed, although it has recently transpired–spoiler alert–that it in fact survived the Last Great Time War between the Daleks and the Time Lords.
So, what do we know about this mysterious world?
Based on a side-by-side comparison with our own planet seen during one recent episode, Gallifrey appears to be a sizeable rocky planet. If it’s similar to or around twice the size of Earth, it’s probably going to have similar geological innards, including a metallic core, a churning mantle, and a crust, Fred Calef, a geoscientist and Mars expert at NASA’s Jet Propulsion Laboratory, explained to Earther.
If it’s between 1.5 and two times the radius of Earth, then it may be more mysterious. Yayaati Chachan, a graduate student of planetary science at the California Institute of Technology, told Earther there’s a dearth of observed exoplanets in this range, meaning we can’t be entirely sure what’s on the inside.
But if Gallifrey is more massive than Earth, “the increased gravity can retain smaller, ‘faster’ molecules,” perhaps hydrogen and helium, according to Calef. An atmosphere with larger molecules in it would also mean that incoming starlight would scatter differently. In this case, it may cast a red hue over everything, matching the burnt orange sky we see in the TV show.
Speaking of starlight, Gallifrey and its neighboring planets orbit two stars–a binary system a bit like Kepler-47, an exoplanet discovered in 2012. In an audiobook, one of Gallifrey’s suns was said to be large and red, but the other isn’t directly alluded to.
Assuming that large red star isn’t just extremely close to the planet, it could be a red giant nearing the end of its life. What of the other? For clues, we can look to the planet’s flora.
The Tenth Doctor referenced trees with silver leaves. Lillian Ostrach, a research physical scientist at the US Geological Survey’s Astrogeology Science Center, told Earther the silver color could come from the absorption of strange metals from Gallifrey’s soil. It could also mean that those plants evolved to absorb a different type of solar radiation than Earth’s green plants do.
Rose Ferreira, a science communicator and student of space biophysics at Empire State College, suggested that silver leaves might absorb infrared light for photosynthesis while reflecting in the ultraviolet. That could mean the other star in the system is a red dwarf, which are known to emit plenty of infrared radiation in general, while blasting out ultraviolet through solar flares.
On the plus side, this type of red giant-red dwarf binary star system can exist; one was spotted in 2010. Sadly, a red dwarf could prove to be problematic for life, as its high-energy solar flares might strip away a planet’s life-sustaining atmosphere.
Beyond their silver hue, the plants on Gallifrey’s surface pose another mystery: The planet doesn’t appear to have any surface water. That makes it likely that it’s extremely hot on Gallifrey. Without a huge carbon sink like Earth’s oceans, it’s likely carbon dioxide would just accumulate in the atmosphere leading to Venus-style runaway global warming.
The better news about Gallifrey’s atmosphere is that it’s probably–perhaps thanks to its weird plants–oxygen-based, something we can assume based on the fact that Time Lords are, as Ostrach put it, “quite similar” to humans in terms of anatomy and physiology. The surface geology is also rust-colored, indicating volcanic rocks whose iron is chemically reacting with the free oxygen in the air.
What else do we know about this hellish hothouse? Well, if it is orbiting a red giant, Gallifrey may be an especially old world. Red giants are stars at the end of their lives that are billions of years old.
Bruce Macintosh, a physics professor and exoplanet researcher at Stanford University, suspects Gallifrey formed in the early days of the universe. If that’s true, it could contain plenty of primitive, lighter elements, like hydrogen, oxygen, carbon and some silicates, resulting in a planet with a thin, rocky shell, and layers of ice and diamond further down. Perhaps, Macintosh said, the planet’s notoriously shiny mountains are rich in diamond, a surface manifestation of a carbon-rich interior.
This early universe hypothesis means that Gallifrey wouldn’t have much iron, as stars wouldn’t have been able to produce much of it yet. This doesn’t quite match up with the rusty-looking rocks at the surface, but maybe the Mars-colored landscape owes its existence to a huge space-borne deliver of iron when, later in its timeline, it was bombarded by iron-rich meteorites.
Between hot, parched conditions and a potentially devastating period of asteroid bombardment, it seems Gallifrey won’t become a popular new tourist destination anytime soon. But in case you’re not entirely convinced to stay the hell away, here’s one last bit of fun trivia.
We know that Gallifrey is–or at least was before it was shifted off to a pocket universe in 2013–located near the center of its galaxy, and that galactic centers contain supermassive black holes. Harriet Brettle, a graduate student of planetary sciences at the California Institute of Technology, suggested that if the system is extremely close to the black hole, this might cause time to run slowly for Gallifreyans compared to those farther from the galactic core thanks to the quirks of general relativity. A visit to Gallifrey, then, could mean that you would return to Earth to discover that your friends and family suddenly much older.
Then again, if Gallifrey is close enough to a black hole for this kind of relativistic effect to kick in, it’s not just wibbly wobbly timey wimey issues you have to be concerned about. According to Macintosh, extremely energetic material being flung off a black hole could “blow away a planet’s atmosphere.”
Sorry, Doctor. You may be brilliant, but your planet is awful.
Correction: An earlier version of this post stated that if Gallifrey was between 1.5 and two times the mass of the Earth its interior could be more mysterious. We should have said 1.5 to 2 times the radius of the Earth. The text has been updated and Earther notes the irony of a scientific error in an article about a fantastical planet.