Exoplanet Guide

Scienceline’s Guide to the Exoplanets: The Twin Dot

2M1207b: The oldest photograph of an exoplanet we have

July 6, 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]

The Twin Dot
2M1207b
Discovered by: Gael Chauvin, et al
Discovered: 2004
Distance from Earth: ~170 light-years
Mass: ~4 Jupiter masses (~1300 Earth masses)
Surface climate: Hot clouds
Habitability for humans: Sadly, but unsurprisingly, little

Orbiting a brown dwarf, round about 170 light-years from the Earth, is a hot Jupiter. As far as exoplanets we’ve discovered go, there’s very little special about 2M1207b. It’s somewhere around four Jupiter masses: large by the standards of our solar system, yes, but hardly standout in a set of known planets that can be more than three times as massive.

Well, the only thing that makes 2M1207b unique is the fact that it’s, potentially, the first exoplanet to have been discovered by direct imaging.

That is to say, it’s likely the first exoplanet that was discovered by astronomers literally taking a picture of it, as if they were in the 19th century and trying to find Neptune through their telescopes.

Prior to 2M1207b, every known exoplanet had been found indirectly: either by radial velocity, by detecting the shift in the star’s velocity from the planet pulling upon it, or by transiting, or seeing the drop in light levels caused by the planet passing between its star and Earth. (Some planets, those orbiting pulsars, had also been discovered by their impact on the pulsar’s regular emission cycle.)

Instead, in 2004, astronomers pointed the Very Large Telescope, located in Chile’s Atacama Desert, at an unassuming brown dwarf named 2M1207. They found not one, but two points of light.

At first, it wasn’t clear what they’d found. They weren’t sure if the second point was anything more than an observational error, or perhaps an optical illusion: the original star’s image merely getting doubled. But later observations, helped along by the Hubble Space Telescope, found that 2M1207b was in fact a second object.

It helps that 2M1207b is quite some distance from its star: about 40 AU, more than the distance between the Sun and Pluto. (The “hot” part of the “hot Jupiter” in this case is mainly from internal heating.) That means telescopes on Earth have a much easier time resolving the two dots separately.

There’s a reason I say that it’s “potentially” the first planet to be found this way. The small star which 2M1207b orbits, and the fact that its size and its mass are still uncertain for lack of further observation, make it uncertain whether 2M1207b is actually a planet — one that formed in place, in that system — or if it was, say, a brown dwarf. It’s possible it formed elsewhere and was drawn into 2M1207’s system.

Unfortunately, there hasn’t been much else astronomers have learned about the planet; there’s not much to make it special aside from its discovery method. Further detections have found traces of water in its atmosphere. As a hot Jupiter, with temperatures in the thousands of degrees, both in Celsius and in Fahrenheit, it’s not habitable.

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