In our solar system, the “ice giants” Neptune and Uranus are much less dense than the rocky Venus and Earth. But astrophysicists on NASA’s TESS mission have found an exoplanet, TOI-849b, which appears to be 40 times more massive than Earth, but just as dense. This illustration shows the exoplanet, UCF-1.01. Like TOI-849b, this exoplanet also orbits close to a star and is like “hot Neptune”. Credit: NASA / JPL-Caltech
NeptuneLarge may be the remnant core of a much larger planet.
The “ice giant” planets Neptune and Uranus are much less dense than rocky terrestrial planets like Venus and earth. Beyond our solar system, many other Neptune-sized planets, orbiting distant stars, appear to have a similarly low density.
Now a new planet discovered by POTSurvey satellite of exoplanets in transit, TESS, seems to counteract this trend. The planet, named TOI-849 b, is the 749th “Object of Interest TESS” identified to date. Scientists saw the planet circling a star about 750 light-years away every 18 hours, and estimate that it is approximately 3.5 times larger than Earth, making it a planet the size of Neptune. Surprisingly, this distant Neptune appears to be 40 times more massive than Earth and just as dense.
TOI-849 b is the most massive Neptune-sized planet discovered to date, and the first to have a density comparable to that of Earth.
“This new planet is more than twice as massive as our own Neptune, which is really unusual,” says Chelsea Huang, a postdoc in MITKavli Institute for Astrophysics and Space Research, and member of the TESS scientific team. “Imagine if you had a planet with the average density of Earth, built up to 40 times the mass of Earth. It’s crazy to think what happens in the center of a planet with that kind of pressure. “
The discovery is reported today in the magazine. Nature. The study authors include Huang and members of the TESS science team at MIT.
A damn Jupiter?
Since launching on April 18, 2018, the TESS satellite has been scanning the heavens for planets beyond our solar system. The project is one of NASA’s Astrophysics Explorer missions and is managed and operated by MIT. TESS is designed to examine almost the entire sky by rotating your view every month to focus on a different patch of the sky as it orbits Earth. While scanning the sky, TESS monitors light from the brightest and closest stars, and scientists look for periodic dips in starlight that may indicate a planet is crossing in front of a star.
Data taken by TESS, in the form of a star’s light curve, or measurements of brightness, are made available to the TESS scientific team, an international group of researchers from various institutes led by MIT scientists. These researchers get a first look at the data to identify promising planetary candidates or TESS objects of interest. These are shared publicly with the general scientific community along with the TESS data for further analysis.
For the most part, astronomers focus their search for planets on the brightest and closest stars TESS has observed. However, Huang and his team at MIT recently had additional time to review the data during September and October 2018, and wondered if anything could be found among the weaker stars. Indeed, they discovered a significant number of transiting dives from a star 750 light years away, and shortly thereafter, confirmed the existence of TOI-849 b.
“Our team doesn’t usually look carefully at stars like this, so this discovery was a happy coincidence,” says Huang.
Follow-up observations of the faint star with a series of ground-based telescopes further confirmed the planet and also helped determine its mass and density.
Huang says the curious proportions of TOI-849 b challenge existing theories of planetary formation.
“We are really puzzled about how this planet was formed,” says Huang. “All current theories do not fully explain why it is so massive at its current location. We don’t expect planets to grow to 40 land masses and then just stop there. Instead, it should continue to grow and end up being a gas giant, like a heat Jupiter, to several hundred land masses. “
One hypothesis that scientists have put forward to explain the mass and density of the new planet is that it was perhaps once a much larger gas giant, similar to Jupiter and Saturn – planets with massive gas envelopes that surround nuclei that are considered as dense as Earth.
As the TESS team proposes in the new study, over time, much of the planet’s gas envelope may have been removed by radiation from the star, which is not an unlikely scenario, since TOI-849 b orbits extremely close to its host star. Its orbital period is only 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 nucleus of a much more massive giant, the size of Jupiter.
“If this scenario is true, TOI-849 b is the only remaining planet core and the largest known gas giant core that exists,” says Huang. “This is something that scientists are excited about because previous theories cannot explain this planet.”
Read the first exposed planetary core discovered to learn more about this discovery.
Reference: “A Planetary Core Remaining in the Warm Neptune Desert” by David Armstrong et al., July 1, 2020, Nature.
DOI: 10.1038 / s41586-020-2421-7
This research was funded, in part, by NASA.
