Exoplanet Guide

Scienceline’s Guide to the Exoplanets: Planet Zero

Gamma Cephei Ab: The first exoplanet discovered on Earth...sort of.

June 23, 2020
A fake planet with "Scienceline's Guide to Exoplanets" labeled across its equator. Other planets zoom past behind it.
Your favorite planets, and you didn't even know they existed. [Credit: Curtis Segarra | CC BY-NC-ND 2.0]

Planet Zero
Gamma Cephei Ab
Discovered by: It’s Complicated™
Discovered: 1988 (sort of)
Distance from Earth: ~45 light-years
Surface climate: Gas
Habitability for humans: No, but it’s fun

What was the first exoplanet discovered?

It sounds simple, but as it turns out, this question doesn’t have an easy answer.

Astronomers have claimed to find evidence of planets around other star systems for a good many decades; it makes little sense, after all, that the Sun would be the only star to have planets. As early as the 1850s, astronomers were grasping at straws to claim evidence of a planet in the 70 Ophiuchi system, a binary system about 16 light-years from Earth. A century later, others claimed to have found gas giants around Barnard’s Star, a red dwarf about six light-years away. (Barnard’s Star does have at least one planet, as it turns out, though it’s not a gas giant, and the earlier claim has been debunked.)

But the first discovery of the current wave of exoplanet discoveries — the one that’s seen thousands upon thousands of exoplanets and the one that’s completely redefined how astronomers think about deep space — probably came in 1988. A group based in Canada announced evidence of an exoplanet in the nearby binary system Gamma Cephei. A British-based group independently came to the same conclusion in 1989.

Of course, it’s not so simple as that. The discovery wasn’t immediately confirmed, and  the astronomers themselves were skeptical. To “find” the planet, they’d used the radial velocity method: essentially, measuring a star moving from that planet gravitationally pulling on it. As you might imagine, that pull is extremely slight, and even then it’s much easier to find only very massive planets. With eighties technology, the Gamma Cephei data wasn’t quite good enough for the “discoverers” to feel confident in what they’d found.

The world would instead have to wait until 1992 for the first confirmed exoplanets (actually two different worlds orbiting a pulsar, PSR B1257+12) and until 1995 for the first confirmed exoplanet around a main-sequence star (51 Pegasi b).

So where does that leave the 1980s events?

Those “discoveries” hadn’t been wrong at all. In 2002, astronomers looked back at the system — one star, an orange giant near the end of its life; its companion, a red dwarf. Orbiting that orange giant, they found, lo and behold, a planet. This, astronomers believe, is what had already been “discovered.”

So what was this planet they’d discovered three separate times? Gamma Cephei Ab, fittingly, is a spectacularly massive gas giant, about 11 times the mass of Jupiter, orbiting at a distance similar to Mars, and taking about two and a half Earth years to finish its orbit. Not a ton else is known about the planet; follow-up studies haven’t done much.

What is known is that, in around 2000 years, the Gamma Cephei system, due to the procession of the Earth’s axis, will be closer to the North Pole in Earth’s sky than Polaris. That means, if you’re in the deep future and you look north…

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