Illustration shows the upper two-thirds of a gas-giant planet, TOI-4600 c, that is similar to Saturn (minus the rings). Cloud bands alternate between light tan, yellow, and darker yellow verging on green.

Illustration of the possible appearance of TOI-4600 c, a "cold Saturn." Credit: NASA/JPL-Caltech

Discovery Alert: A Long Year for a ‘Cold Saturn’

The discovery: Two giant planets comparable to our own system’s Saturn orbit a star not unlike our Sun some 700 light-years away. The outer planet has the longest year – 483 days – of any found so far by NASA’s TESS (the Transiting Exoplanet Survey Satellite). It’s also among the coldest.

Key facts: The two planets, TOI-4600 b and c, could prove important to astronomers who investigate how large, gaseous planets form and evolve. And they begin to fill a gap in knowledge between gas giants like Jupiter and Saturn in our solar system, and “hot Jupiters” (as well as “warm Jupiters”) elsewhere in our galaxy.

Details: The decades-long hunt for exoplanets – planets around other stars – has so far yielded more than 5,500 that are confirmed to be scattered across the Milky Way, which likely contains hundreds of billions. But the prevailing detection method turns up relatively few “long period” planets, those with years lasting 50 days or more. This method, seeking “shadows,” much more easily reveals planets orbiting their stars closely, with far shorter years. The search for shadows, called the transit method, captures the tiny dip in starlight as an orbiting planet crosses the face of its star.

Spaceborne telescopes like TESS that rely on this method are responsible for the vast majority of exoplanet detections. But the longer a planet’s orbit, the harder it is for TESS to catch it transiting its star. Still, in a study published in September 2023, an international team of scientists using TESS data determined that TOI-4600 b and c have long-period orbits: 83 days for planet b, 483 for planet c (a year that’s a bit longer than Earth’s).

These orbits might not sound very impressive compared to the gas giants in our solar system. For Jupiter, one trip around the Sun takes 12 years; a “year” on Saturn equals more than 29 years on Earth. But because fewer long-period exoplanets transiting their stars have been detected, TOI-4600 b and c could prove to be a gold mine of data. While space telescopes have been able to measure some atmospheric components of hot and warm Jupiters, TOI-4600 b and c offer the rarer prospect of revealing atmospheric ingredients of “temperate” gas giants – those without scorching atmospheres.

Fun facts: “Temperate” is, of course, a relative term. If you’re looking for vacation spots, it’s best to leave these two planets off the list. TOI-4600 b, a bit smaller than Saturn at nearly seven times the width of Earth, has an estimated atmospheric temperature of 165 degrees Fahrenheit (74 Celsius). Planet c, about the size of Saturn at more than nine times the width of Earth, has an estimated temperature of minus 116 Fahrenheit (minus 82 Celsius). That’s among the coldest exoplanets TESS has discovered so far.

The discoverers: An international team led by astronomer Ismael Mireles of the University of New Mexico published their paper on the two planets, “TOI-4600 b and c: Two Long-period Giant Planets Orbiting an Early K Dwarf,” in “The Astrophysical Journal Letters” in September 2023.

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