NASA's Transiting Exoplanet Survey Satellite (TESS) has discovered a set of three worlds orbiting a peaceful star located a mere 73 light-years from Earth in the constellation of Pictor. The exoplanets, which are among the smallest and closest known, are a tantalizing target for scientists hoping to understand how different-sized planets come to form, and may even be a future hunting ground for extraterrestrial life.
TESS was launched on April 18, 2018 with a lofty goal – to bring to light thousands of worlds that orbit within the Milky Way, and are hitherto unknown to us. The telescope boasts four identical cameras that combine to observe a "sector" of the night sky spanning 24 x 90 degrees.
The light from each of the many stars contained in a sector is observed continuously for a period of 27 days. A dip of light from one of these distant light sources could indicate the presence of a planet transiting across the surface of its parent star relative to Earth. Candidate exoplanets detected by TESS's wide eyes can then be targeted by ground or space-based telescopes for follow-up observations.
The newly-discovered planets, which are detailed in a paper published today in the journal Nature Astronomy, number among the smallest and closest exoplanets discovered to date. The initial TESS data was supplemented by multi-wavelength photometry, spectroscopy, and high-resolution digital imagery collected by ground-based telescopes.
After vetting the data, it was concluded that three worlds were orbiting the nearby star TOI 270, which had been so named because it was the 270th object of interest that TESS has identified since it began science operations.
The trio of newfound worlds orbit around an M-dwarf star that has already experienced its youthful energetic phase, and now shines with a steady light. The calm nature of the star, along with its high luminosity thanks to its relative proximity relative to Earth, makes TOI 270 an excellent prospect for exoplanet scientists.
Planet TOI 270 b – the innermost world – is thought to be a rocky super-Earth with a radius roughly 1.25 times that of our planet. Meanwhile the outermost two planets, TOI 270 c and d, are likely gas-dominated sub-Neptune worlds. Planet c, which occupies the middle orbit of the three worlds has a radius equivalent to 2.4 times that of Earth, while Planet d is slightly smaller.
Based on the available data, the team believe that all three worlds are tidally locked. This means that they only rotate once per orbit, and always present the same side to their parent star in the same way that the Moon always presents the same face to the Earth.
The two Neptune-like exoplanets are similar in terms of radius, and occupy a kind of intermediate size-range. This makes TOI 270 very different from our Sun, which has no intermediate planets orbiting it. Instead, our solar system is occupied by two extremes – the smaller rocky planets such as Mercury and Venus, and the enormous gassy giants Jupiter, Saturn and Neptune.
The researchers hope that an analysis of the sub-Neptune planets could help shed light on whether smaller rocky planets like our Blue Marble follow similar evolutionary paths to the larger planets, or whether these formative processes diverge.
"There are a lot of little pieces of the puzzle that we can solve with this system," says Maximilian Günther, a postdoc in MIT's Kavli Institute for Astrophysics and Space Research and lead author the study detailing the discovery. "You can really do all the things you want to do in exoplanet science, with this system."
Alongside being an excellent laboratory for studying the evolution of planets, TOI 270 may also be capable of hosting life.
The team had initially believed that the outer sub-Neptunian world TOI 270 d orbited within the habitable zone of its parent star – a region of space where liquid water could theoretically exist on a planet's surface. However, further analysis revealed that the exoplanet's atmosphere was likely too dense, and that this would have triggered a runaway greenhouse effect, which would have rendered the surface too hot to sustain life.
Whilst the surface is a no-go, the upper atmosphere of the sub-Neptune world may have a temperature that could allow extremophile organisms to exist. Furthermore, the researchers note that there could be yet more worlds orbiting TOI-270 that have yet to be discovered, and that one could potentially exist in the star's habitable zone.
Of the three confirmed planets, TOI 270 b – the innermost planet – has an orbital period of roughly three Earth-days. Planet c takes a more leisurely five days to orbit its star, while Planet d takes 11. It was discovered that the three alien worlds orbit in a pattern known to astronomers as a "resonant chain."
Periodically, as the exoplanets move through their orbits, they line up with each other relative to their star, and their gravitational forces influence, or resonate with one another.
"For TOI-270, these planets line up like pearls on a string," Günther says. "That's a very interesting thing, because it lets us study their dynamical behavior. And you can almost expect, if there are more planets, the next one would be somewhere further out, at another integer ratio."
Moving forward, the researchers plan to bring more powerful telescopes to bear on TOI 270, including the future James Webb Space Telescope.
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