TESS finds the first two mini-Neptune exoplanets

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In discovering three candidate exoplanets orbiting a nearby red dwarf star, the space telescope TESS has found the first two that are sized somewhere between the rocky Earth-sized planets and the larger Neptune-sized gas giants.

The innermost planet, TOI 270 b, is likely a rocky world about 25% larger than Earth. It orbits the star every 3.4 days at a distance about 13 times closer than Mercury orbits the Sun. Based on statistical studies of known exoplanets of similar size, the science team estimates TOI 270 b has a mass around 1.9 times greater than Earth’s. Due to its proximity to the star, planet b is an oven-hot world. Its equilibrium temperature — that is, the temperature based only on energy it receives from the star, which ignores additional warming effects from a possible atmosphere — is around 490 degrees Fahrenheit (254 degrees Celsius).

The other two planets, TOI 270 c and d, are, respectively, 2.4 and 2.1 times larger than Earth and orbit the star every 5.7 and 11.4 days. Although only about half its size, both may be similar to Neptune in our solar system, with compositions dominated by gases rather than rock, and they likely weigh around 7 and 5 times Earth’s mass, respectively.

All of the planets are expected to be tidally locked to the star, which means they only rotate once every orbit and keep the same side facing the star at all times, just as the Moon does in its orbit around Earth.

Planet c and d might best be described as mini-Neptunes, a type of planet not seen in our own solar system. The researchers hope further exploration of TOI 270 may help explain how two of these mini-Neptunes formed alongside a nearly Earth-size world.

The star is only 73 light years away.

Need I say that our level of knowledge about solar system formation is tiny at this point, and that any models any theorist creates should merely be seen as scratchpad first approximations, useful only in guiding future research and not to be taken too seriously.



  • David Eastman

    Assuming they’ve identified these exoplanets and their characteristics with any accuracy, those have got to be some pretty interesting places. A tidally locked gas giant well inside Mercury’s orbit has got to have some amazing stuff going on in that atmosphere. Would it sort itself into layers based on the solar input with certain gasses facing the sun and other gasses hiding in the shadow? Or is the insolation from that dwarf star so low that even at 0.05au it’s just not that dramatic?

  • MDN

    Only 73 Light Years = Only 429 TRILLION miles : )

    Close astronomically, but still a very, very, very, VERY long way away. Maybe one of our digital progeny will visit it in a millennia or so.

  • Scott M.

    It’s fascinating how many of these hot gas giants we’ve found. Is this simply a case of selection bias due to our method of observation, or is there something about planetary system formation that we don’t understand?

  • wayne

    I rather think it’s both.
    (I seem to recall watching a lecture on TESS recently wherein selection-bias was touched upon in some depth, if I can find it I’ll post it.)

  • Scott and Wayne: The answer is both. We don’t know enough, and we have a considerable selection bias.

  • wayne

    Mr. Z.,
    totally thinking out loud– we really can’t explain the formation of our own solar system, can we? I’m just arm-chair familiar with the theories, but we only really have a sample size of 1, ya know!?
    That having been said, “we’re living in the future.” This is all amazing stuff.

    — highly informative:
    Initial Exoplanet Discoveries from TESS
    Space Telescope Science Institute
    January 2019
    (content starts about 20 minutes in)
    ” Dr. Scott Fleming will share several of the first exciting discoveries, plus detail about how the observatory is designed and how it compares to NASA’s Kepler/K2 mission. All of the data from TESS is available in the Barbara A. Mikulski Archive for Space Telescopes (MAST), which is based at the Space Telescope Science Institute.”

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