A trio of exoplanets may challenge what we know about planetary formation.
A new discovery adds to the growing menagerie of exoplanets. These days, word of a new exoplanet discovery raises nary an eyebrow. To date, the current number of known exoplanets beyond our solar system stands at 6,148 worlds and counting.
But a recent study out of the University of Liège in Belgium titled *Two Warm Earth-sized Planets and an Earth-sized Candidate in the Binary System TOI-2267* shows just how strange these worlds can be.
The finding was made by detection software named SHERLOCK (which is a tongue twister of an acronym even by astronomical standards, as the Search for Hints of Exoplanets from Light curves Of space-based Seekers) looking at data gathered by the NASA TESS (the Transiting Exoplanet Survey Satellite) mission.
“The first hint that we were looking at something truly new came from our own planet-search software, which revealed two additional planets in the system that NASA’s TESS mission had not identified,” Sebastián Zúñiga-Fernández (University of Liège) told *Universe Today*. “We spent many hours discussing whether these signals could really correspond to planets, since they implied a completely new kind of system architecture. When we finally confirmed their existence with the SPECULOOS network telescopes, it was an incredibly rewarding moment; we had proven that extremely compact binary systems can indeed host Earth-sized planets! The discovery is perfectly in line with the name of our research group in Liège, Belgium, led by Michaël Gillon: EXOTIC (EXOplanets in Transit: Identification and Characterization).”
The bizarre worlds of TOI-2267 are located 190 light-years distant in the direction of the northern constellation of Cepheus the King, near Right Ascension 4 hours 20’, Declination +84 degrees 54’ north.
The trio orbits a pair of M-type red dwarf stars, which are in turn 8 Astronomical Units (AUs) apart, slightly closer than the distance between the Sun and Saturn.
TOI-2267b has an orbital period of about 2.28 days, while TOI-2267c has a period of about 3.49 days, and their distances are approximately 0.0205 AU and 0.0263 AU respectively from their host star. The third planet, TOI-2267.02, is an exoplanet candidate with a period of about 2.03 days with a distance of approximately 0.0124 AU from its host star.
0.02 AU is equal to not quite 2 million miles; for context, the Moon is a quarter of a million miles from Earth, and Callisto is just over a million miles from Jupiter. We’re talking two solar systems in miniature, locked in a leisurely embrace. Standing on the surface of TOI-2267b, you’d see a bright red dwarf primary dominating the sky (it’s probably tidally locked) with a fainter but still prominent red dwarf companion nearby.
“It’s the first binary system where we see planets passing in front of—or transiting—both stars. We knew of binary systems with planets, but in the vast majority of cases, the planets would orbit just one of the stars,” says Zúñiga-Fernández. “In the very rare cases where planets were found around both, the stars were so far apart that they acted like two separate systems. TOI-2267 is a compact system where the two stars are relatively close together (approximately the distance between the Sun and Jupiter), and yet each has its own transiting planets. This is a brand-new architecture for a planetary system, making it a unique laboratory to study how planets form and evolve in the complex gravitational environment of a double star.”
The three-body problem says that planetary systems around multiple star pairs should be unstable over the long term. It seems that we’re already seeing some exceptions to the rule, in the trio of worlds in the TOI-2267 system.
TESS is an all-sky survey mission, looking for transiting exoplanets. Launched in 2018, TESS has far outlived its planned 2-year nominal mission. To date, TESS has discovered 7,655 candidate exoplanets. The future for exoplanet science may lie beyond programs like SHERLOCK, TRAPPIST, and SPECULOOS, which isn’t the Belgian-spiced cookie of the same name, but the Search for habitable Planets Eclipsing Ultra-cOOl Stars.
“We have largely exhausted the follow-up observations possible with our TRAPPIST and SPECULOOS telescopes,” says Zúñiga-Fernández. “The next step requires the capabilities of larger facilities, such as the James Webb Space Telescope (JWST) or ground-based telescopes with over 6-meter diameters like the Gran Telescopio Canarias (GTC). The precision of these premier instruments is essential to perform more accurate transit timing measurements—looking for TTVs that can reveal the planets' masses—and to potentially study their atmospheres, should they exist.”
Exoplanet science is a lesson in how common—or exceptional—the story of our solar system may be in the wider drama of the Universe. We’re now here, orbiting a solitary star, looking out at strange new worlds orbiting multiple stars that may be much more common than our own.