Also this week, we’ve discovered a gigantic cold planet, the first exoplanet of a pair

This week, astronomers from the Carnegie Institution of Washington announced a massive exoplanet — a planet that is too big to be a gas giant — orbiting two suns.

As the story posted by CU-Boulder’s Mile High Astronomy website points out, this is not an Earth-to-dwarf planet, as a number of astrophysicists have come to expect. Instead, the planet is a “water world” about as large as Jupiter and up to 10 times the mass of Earth. It is the first time that water worlds have been found in pairs.

Pairs of suns form when massive planets transfer their mass as they orbit their parent stars. Einstein’s special theory of relativity suggests that if two planets spin in tandem (ie, orbit each other), the mass of each one will be distributed unevenly (eg, one island stretches slightly more than the other). The strong gravitational interactions between the planets may alter this mass distribution.

The 2,100-light-year-away KELT-1* orbit is 16 times closer to its star than Earth is to the sun. That means that this planet — whatever its composition — orbits the host star very rapidly.

It can therefore be thought of as a kind of giant, cold planet:

“This is the fourth known water world in two orbits. Previous water worlds were typically too hot or too small to be habitable, but KELT-1* is the first water world to be seen as it appears in its host star’s spectrum (which is that distinctive signature of the star’s light as it reaches us from the distant sky). And it shows the definite signature of a water world whose dense core likely contains water, as observed by the glow from its atmosphere.”

“The combined mass of KELT-1* and its companion stars is 10 billion times more massive than that of the sun. It is essentially three planets — a gas giant, a warm, liquid world, and a cool, solid Earth — all orbiting in a singular orbit.”

This news follows the discovery of water worlds that orbit between four and seven solar masses of our sun — in ways that make them cold and rocky. The atmospheres of these exoplanets, while still heating up to a degree or two above the freezing point of water, are typically carbon-containing — just like Earth. They are so many that it might be possible to create a group of “zodiacal discs” — stable, dust-laden swarms orbiting around gas giants.

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