A strange ice giant that can’t be explained by previous theory has been discovered by NASA Scientists using the exoplanet-sleuth Transiting Exoplanet Survey Satellite (TESS, which had “first light” on Aug. 7, 2018). The planet, TOI-849 b, is the most massive Neptune-sized planet detected to date, and the first to have a density that is comparable to Earth. The odd proportions of planet orbits a star about 750 light years from Earth every 18 hours, and is 40 times more massive.
“We’re really puzzled about how this planet formed,” says Chelsea Huang, a postdoc in MIT’s Kavli Institute for Astrophysics and Space Research, and a member of the TESS science team. “All the current theories don’t fully explain why it’s so massive at its current location. We don’t expect planets to grow to 40 Earth masses and then just stop there. Instead, it should just keep growing, and end up being a gas giant, like a hot Jupiter, at several hundreds of Earth masses. It’s quite crazy to think what’s happening at the center of a planet with that kind of pressure.”
Since its launch on April 18, 2018, the TESS satellite has been scanning the skies for planets beyond our solar system. The project is one of NASA’s Astrophysics Explorer missions and is led and operated by MIT. TESS, which is predicted to discover a handful of habitable alien planets, is designed to survey almost the entire sky by pivoting its view every month to focus on a different patch of the sky as it orbits the Earth. TESS uses the “transit method,” which looks for slight dips in star brightness caused when an orbiting planet crosses the star’s face from TESS’ perspective, similar to NASA’s Kepler space telescope, which discovered about 70% of the more than 4,000 or so known alien worlds.
“Tess Object of Interest”
Data taken by TESS, in the form of a star’s light curve, or measurements of brightness, is first made available to the TESS science team, an international, multi-institute group of researchers led by scientists at MIT. These researchers get a first look at the data to identify promising planet candidates, or TESS Objects of Interest (TOI). These are shared publicly with the general scientific community along with the TESS data for analysis.
Hidden in the Data
For the most part, astronomers focus their search for planets on the nearest, brightest stars that TESS has observed. Huang and her team at MIT, however, recently had some extra time to look over data during September and October of 2018, and wondered if anything could be found among the fainter stars. Sure enough, they discovered a significant number of transit-like dips from a star 750 light years away, and soon after, confirmed the existence of TOI-849 b.
“Stars like this usually don’t get looked at carefully by our team, so this discovery was a happy coincidence,” Huang says.Follow-up observations of the faint star with a number of ground-based telescopes further confirmed the planet and also helped to determine its mass and density.
Previous Theories Can’t Explain this Planet
One hypothesis scientists have come up with to explain the new planet’s mass and density is that perhaps it was once a much larger gas giant, similar to Jupiter and Saturn — planets with more massive envelopes of gas that enshroud cores thought to be as dense as the Earth.
As the TESS team proposes in the new study, over time, much of the planet’s gassy envelope may have been blasted away by the star’s radiation — not an unlikely scenario, as TOI-849 b orbits extremely close to its host star. Its orbital period is just 0.765 days, or just over 18 hours, which exposes the planet to about 2,000 times the solar radiation that Earth receives from the sun. According to this model, the Neptune-sized planet that TESS discovered may be the remnant core of a much more massive, Jupiter-sized giant.
“If this scenario is true, TOI-849 b is the only remnant planet core, and the largest gas giant core known to exist,” says Huang. “This is something that gets scientists really excited, because previous theories can’t explain this planet.”
The Daily Galaxy, Max Goldberg, via MIT