Illustration of the possible appearance of TOI-4600 c, a "cold Saturn." NASA/JPL-Caltech
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.
Discovery Alert: A 'Super-Earth' in
the Habitable Zone
Discovery Alert: Earth-sized Planet Has a ‘Lava Hemisphere’
Source: Discovery Alert: a Long Year for a 'Cold Saturn' (nasa.gov)
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