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TOI 700 d: A Possible Habitable Zone Planet

Among the discoveries announced at the recent meeting of the American Astronomical Society in Hawaii was TOI 700 d, a planet potentially in the habitable zone of its star. TOI stands for TESS Object of Interest, reminding us that this is the first Earth-size planet the Transiting Exoplanet Survey Satellite has uncovered in its data whose orbit would allow the presence of liquid water on the surface. The Spitzer instrument has confirmed the find, highlighting the fact that Spitzer itself, a doughty space observatory working at infrared wavelengths, is nearing the end of its operations. Thus Joseph Rodriguez (Center for Astrophysics | Harvard & Smithsonian):

“Given the impact of this discovery – that it is TESS’s first habitable-zone Earth-size planet – we really wanted our understanding of this system to be as concrete as possible. Spitzer saw TOI 700 d transit exactly when we expected it to. It’s a great addition to the legacy of a mission that helped confirm two of the TRAPPIST-1 planets and identify five more.”

Image: The three planets of the TOI 700 system, illustrated here, orbit a small, cool M dwarf star. TOI 700 d is the first Earth-size habitable-zone world discovered by TESS. Credit: NASA’s Goddard Space Flight Center.

TOI-700 is an M-dwarf star in the constellation Dorado, a southern sky object whose mass and size are roughly 40 percent that of the Sun, with half the Sun’s surface temperature. Remember that TESS monitors sky sectors in 27-day blocks, a period lengthy enough to spot the changes in stellar brightness that mark the transit of a planet across the star’s face as seen from Earth.

It’s interesting to note how any misclassification of stellar type can confound our conclusions about a transiting planet. In this case, the star was originally thought to be closer in size and type to the Sun, which would have meant planets that were larger and hotter than we now know are there. Correcting the problem revealed what looks to be a very interesting world.

“When we corrected the star’s parameters, the sizes of its planets dropped, and we realized the outermost one was about the size of Earth and in the habitable zone,” said Emily Gilbert, a graduate student at the University of Chicago. “Additionally, in 11 months of data we saw no flares from the star, which improves the chances TOI 700 d is habitable and makes it easier to model its atmospheric and surface conditions.”

Video: NASA’s Transiting Exoplanet Survey Satellite (TESS) has discovered its first Earth-size planet in its star’s habitable zone, the range of distances where conditions may be just right to allow the presence of liquid water on the surface. Scientists confirmed the find, called TOI 700 d, using NASA’s Spitzer Space Telescope and have modeled the planet’s potential environments to help inform future observations. Credit: NASA’s Goddard Space Flight Center.

What we now know about TOI 700 is that there are at least three planets here, with TOI 700 d being the outermost and the only one likely to be in the habitable zone. The planet is in a 37 day orbit and receives 86 percent of the insolation that the Sun gives the Earth. Here again we look to Spitzer, for its data allowed researchers not only to confirm the existence of TOI 700 d but also to tighten the constraints on its orbital period by 56% and its size by 38%. Further observations from the Las Cumbres Observatory network also tightened the orbital period.

The other worlds in the TOI 700 system are TOI 700 b, about Earth size and probably rocky, orbiting the star every 10 days, and TOI 700 c, 2.6 times larger than Earth and in a 16 day orbit. As to the intriguing TOI 700 d, let’s keep in mind that it’s relatively close at just over 100 light years, making it a potential target for follow-up observations by future space observatories, although not the James Webb Space Telescope, as I’ll explain in a moment. TOI 700 d is also likely to be, along with its planetary companions, in tidal lock with the star, keeping one side constantly in daylight, the other in perpetual night.

This nearby star appears to have low flare activity, adding to the potential that if TOI 700 d is truly in its habitable zone, any life developing there would not have to cope with severe doses of UV and X-rays. We should be able to get radial velocity information on this system that could firm up our assumptions about the composition of the three planets by determining their density when contrasted with the transit data that gives us their size.

For the time being, researchers at NASA GSFC have modeled 20 potential environments for TOI 700 d, using 3D climate models that consider various surface types and atmospheric compositions. Led by Gabrielle Engelmann-Suissa (a USRA visiting research assistant at GSFC), the team simulated 20 spectra for the 20 modeled environments. Dry, cloudless worlds and ocean-covered surfaces showed the range of possibilities. Such simulations can be of high value, as the paper on this modeling points out:

While the detection threshold of the spectral signals for this particular planet are most likely unfeasible for near-term observing opportunities, the end-to-end atmospheric modeling and spectral simulation study that we have performed in this work is an illustrative example of how global climate models can be coupled with a spectral generation model to assess the potential habitability of any HZ terrestrial planets discovered in the future, as we have done here with the exciting new discovery, TOI-700 d. With more discoveries on the horizon with TESS and ground-based surveys, we hope that this methodology will prove useful for not only predicting the observability of HZ planets but also for interpreting actual observations in the years to come.

The paper on the modeling is one of three describing the work on TOI 700 d. It makes clear that the noise floor of JWST, which takes into account instrument noise aboard the telescope, makes it unlikely the observatory will be able to characterize TOI 700 d. Similarly, direct imaging even by next-generation extremely large telescopes (ELTs) is challenging. Thus the paper’s conclusion: “Significant characterization efforts will therefore require future space-based IR interferometer missions such as the proposed LIFE (Large Interferometer For Exoplanets) mission.”

It’s good, then, to have TOI 700 d in our catalogs, but it’s not going to be the first exoplanet whose atmosphere we can probe for potential biosignatures.

Three papers describe this work. They are Gilbert et al., “The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System,” submitted to AAS Journals (preprint); Rodriguez et al., “The First Habitable Zone Earth-Sized Planet From TESS II: Spitzer Confirms TOI-700 d,” submitted to AAS Journals (preprint); Engelmann-Suissa et al., “The First Habitable Zone Earth-sized Planet from TESS. III: Climate States and Characterization Prospects for TOI-700 d,” submitted to the Astrophysical Journal (preprint).


Comments on this entry are closed.

  • Laintal January 9, 2020, 16:08

    Hi Paul

    Very interesting reading on yet another amazing discovery, and thanks for the links to the papers too.

    There is a chance this planet might not be tidally locked as it could be far enough away from its large host star to escape tidal locking

    Now off to check out the papers.

    Cheers Laintal

  • Bruce D. Mayfield January 9, 2020, 18:33

    If it gets only 86% of Earth’s insolation why is it depicted in the system illustration as being so close to the inner edge of the green habitable zone?

    • Enzo January 9, 2020, 22:19

      Perhaps people are finally starting to use a more realistic HZ (inner edge ~1.1 solar flux) rather than the optimistic ones (like Venus type of solar flux). There was a time when announcements of an “earth-like planet in the HZ” were regular. So much so that every time I had to check “HZ, eh ? But which one ?” and, more often than not, it was like 1.5-2x.
      Moreover, consider how thin earth’s atmosphere is compare to what a planet of similar size can potentially hold (i.e. Venus). This fact makes me more comfortable with worlds that are further out, especially if they are larger than earth.

    • Matthew Picio January 10, 2020, 9:29

      That inner boundary isn’t due to temperature as it would be with a Type G star. With M class dwarves the inner boundary is more frequently the point at which an orbiting body will become tidal locked. Generally that’s not considered habitable, as any atmosphere would have boiled off on the starward side and frozen or sublimated off on the backside.

      • Ashley Baldwin January 10, 2020, 14:42

        The habitable zone is not defined by the presence or absence of tidal locking . As described by Kasting et al in a seminal 1993 article, it represents the area surrounding a star where liquid water can exist on a planetary ( or lunar) surface .

        This is largely determined by the energy flux from the star. The inner boundary representing that point at which the flux leads any atmosphere to become subject to ‘runaway greenhouse’ ( loss of water and superheating via photolysis of water vapour in the stratosphere ) . Tidal locking can actually help drive the hab zone inwards with the postured formation of thick cloud at the ‘sub stellar point’ ( nearest bit of the planet to the star ) which reflects incident starlight and drives down surface temperature .

        The outer boundary is that point at which the maximum greenhouse effect achievable via the addition of CO2 is reached – after which it’s addition causes condensation into clouds which reflect stellar energy back into space . Cooling the planet .

        So the habitable zone is dependent on both stellar flux and the energy distribution of that flux which will vary across stellar spectra. M dwarfs emit far more of their energy in low energy infrared wavelengths – though these are better absorbed by terrestrial atmospheres .

        Any star with a mass of up to 0. 75 Msun will have some or all of its hab zone within the tidal lock zone within a 5 Billion years time span ( including probably all M dwarfs ) – though planetary resonances and thick atmospheres might mitigate .

        Tidal locking or ‘synchronisation’ isn’t now seen as an absolute barrier to habitability with both planet wide oceans and/or thick atmospheres simulated as able to transfer heat from the star exposed side to the dark side.

  • ljk January 10, 2020, 0:04

    Puerto Rico’s Iconic Arecibo Observatory Closed by Major Earthquake

    By Meghan Bartels

    9 hours ago

    HONOLULU — Staff at Puerto Rico’s iconic Arecibo Observatory are monitoring the facility for potential damage in the midst of a spate of earthquakes rocking the island.

    The strongest of those quakes was a 6.4 temblor early in the morning of Tuesday (Jan. 7). An initial survey conducted by drone after that event found no damage to the massive radio dish or the equipment above it, an Arecibo Observatory representative said here at the 235th meeting of the American Astronomical Society on Tuesday (Jan. 7).


  • P January 10, 2020, 0:36

    I think that might have something to do with the very different spectral energy distribution of an M star?


  • Ronald January 10, 2020, 8:14

    And talking about suitable stars for habitable planets, this just came out on NASA’s Hubble site (and was presented at the 235th meeting of the American Astronomical Society):


    Note: they forgot the word habitable here, but without it, this statement would probably be untrue.
    Guinan and Engle find that orange K-stars are probably the most suitable candidates for planets with life, because they, on the one hand, don’t have the flaring problem of M-dwarfs, and, on the other hand, they live a stable main sequence life (much) longer than ‘our’ type G stars.

    Of course, this reminds us again of the concept of super-habitability by Armstrong and Heller (2014), who also identified K-stars as the most suitable stars for long-term habitability.

    Worthy of a separate post, Paul.

    • Paul Gilster January 10, 2020, 11:32

      Yes, it’s in queue.

  • Ashley Baldwin January 10, 2020, 11:02

    The near to mid infrared wavelengths at which an M2/3 spectral class star would emit its energy will be better absorbed by the terrestrial style atmosphere of any erstwhile habitable planet. Such wavelengths are also less likely to be reflected back into space ( than solar visible light) by any polar icecaps too. A lower ‘ice albedo fraction.’

    I guess too that as the planet is still very closer to its star that gravitational heating might play a role as well. Dependent on the eccentricity of the orbit. This along with heat generated internally from the potentially larger and hotter core of a 2.3 Me planet too.

    It’s worth noting that the Earth itself with just 14 % greater insolation, is also close to the inner conservative habitable zone of the solar system .

  • Harold Shaw January 10, 2020, 15:04

    Could tidal locking increase the likelihood that a planet has exposed continents by locking water in glaciers on the cool side of the planet? Granted, the range for water content where this would occur would be narrow but could being tidally locked increase chances for abiogenesis?

  • Geoffrey Hillend January 11, 2020, 19:56

    Yes, an a 2.3 Me might have a thick atmosphere due to a higher gravity and a thicker atmosphere larger than one Earth atmosphere which would have a increased greenhouse effect so that might drive up the temperatures higher especially since the exoplanet is at the inner edge of the life belt. There might be more evaporation of water, yet higher pressure means a higher boiling point and evaporation temperature. Maybe with a large telescope we can get the spectra of some of these super Earths.

    • Alex Tolley January 12, 2020, 15:33

      Isn’t the HZ boundary independent of the planet’s size? It assumes the best case to maintain liquid water on the surface without allowing a greenhouse effect while teh star in its current main sequence position. Similarly for teh outer edge except that GHG gases must remain gaseous to maintain the GH effect to prevent a frozen world. So Earth’s orbit is inside our HZ (obviously), Venus’s orbit outside the inner edge, and Mars’s orbit possibly inside or outside the outer edge depending on some assumptions. Over time, Earth will slowly slip outside the inner edge, and Mars will enter the outer edge.

  • Ashley Baldwin January 12, 2020, 14:34

    Time for a “Drew ex Machina” Planetary habitability check on TOI 700d AND the recently RV confirmed Gliese 229Ac ?

    The latter seems to have gone under the radar ( if not the spectrograph ) thanks to the headline TESS findings – but is equally as exciting – especially given its relative proximity at just 19 light years. Making it a prime target for direct imaging by the ELT .

    The presence of Gliese 229 Ab adds to the relevance of this system . I suspect this “super Neptune” will become the most accurately characterised planet outside the solar system over the next decade thanks to Gaia, the ELT and hopefully WFIRST too.

    • Andrew LePage January 13, 2020, 0:46

      I do have a “Habitable Planet Reality Check” on TOI 700d in the works and hope to have it out by mid-week. As for Gliese 229Ac, it does appear to orbit in the HZ but given its Mpsini of ~7 times that of the Earth, it is probably a volatile-rich mini-Neptune with poor prospects of being habitable in the Earth-like sense. Still, there have been a *LOT* of newly announced (but not necessarily habitable) exoplanets orbiting nearby red dwarfs over the last several months (in addition to the Gliese 229Ac and others in the just-published Feng et al.) and I have been considering writing about these either collectively or as part of a new series.

      • Ashley Baldwin January 13, 2020, 10:19

        Great. Thanks Andrew. Looking forward to it as ever.

        Though not habitable, I would posit that Gliese 229Ab has a big scientific future ahead of it. By my crude estimate it receives around 16 % the stellar flux of Earth which is still significantly more than Jupiter. Gently chilled or lukewarm ?

        As b will no doubt be one of the first imaging targets for METIS on the ELT it’s possible precision RV spectroscopy -of the planet itself – might even reveal the first bone fide exomoon.

        Taken together the whole Gliese 229A system is very interesting and will assuredly receive a-lot of scrutiny given its convenient proximity.

        • Andrew LePage January 13, 2020, 17:29

          Oh, I absolutely agree!!! While much of my recent exoplanet-related writing has centered on potentially habitable worlds (along with a lesser number dealing with nearby planetary systems habitable or not), there is still a lot to learn from further observations of *all* exoplanets especially those of nearby stars (e.g. GJ 229) which can be observed by a range of improving techniques and emerging technologies.

  • Brian Schmidt January 12, 2020, 23:00

    The paper authors make clear in the papers and on Twitter that tidal locking as a concept includes orbit-spin resonances, like the case of Mercury, and they modeled two such resonances as part of their 20 models. I was glad to learn that astronomers aren’t ignoring resonances when discussing tidal locking, something that could make a big difference in the types of climate.

    I think the broader discussion of tidally-locked exoplanets does usually ignore the possibility of resonances, so hopefully that will change.

    • Ashley Baldwin January 13, 2020, 18:14

      Good point. Leconte et al, 2015 also elegantly show how various bar CO2 atmosphere can also help resist synchronisation too. Not necessarily that dense either , certainly within terrestrial style limits.

  • Geoffrey Hillend January 13, 2020, 1:37

    Allex Tolley, the greenhouse effect depends on the atmospheric pressure of the planet and how much greenhouse gas is in it’s atmosphere. The Earth has always had carbon dioxide in the last 2 billion years, but the amount has fluctuated widely. There were two snowball Earth periods where rapid weathering with the Earth continents and mountains sucked up all the CO2 out of the atmosphere. The last snow ball Earth period was 715 million years ago. The whole Earth was frozen including the oceans or at least the surface of the oceans in some places. The carbon cycle is responsible for this process.
    Carbon dioxide CO2 traps the thermal infra red radiation from escaping into space. The Sun light hits the ground and warms it up and due to the conservation of energy it is re radiated in thermal infra red light. The CO2 molecules in the air vibrate and rotate when they absorb infra red light radiated from the warm ground.which and the vibration and rotation of molecules the same as heat. The more CO2 added to the atmosphere, the more rotating CO2 molecules, the more the air warms up and the ground warms up in response.

    The carbon cycle. Volcanoes expel CO2. A very active volcanic period will produce a lot of CO2. The climate warms up and with higher temperatures there is more evaporation of sea water and more water vapor in the atmosphere for clouds. It rains a lot more in a hot house Earth. The rain takes the CO2 out of the air and combines it with soil pore water to make carbonic acid which combines with calcium silicate to become silicate and calcium carbonate(limestone). These in water flow into rivers and journey to the sea and the limestone builds up on the bottom of the sea. Also the rain falls in the ocean and takes more CO2 out of the atmosphere.

    The snow ball Earth periods, the first 715 million years ago are important because they are the result of when the carbon cycle weathering, photosynthesis removed most of the CO2 out of the air because there was a period of low or no volcanism. Without CO2, there is little greenhouse effect to keep the Earth warm. The snow ball Earth period lasted for 120 million years until there was enough volcanism and CO2 build up to change the climate. The coldest periods in history, the snow ball Earth periods were followed by the hottest periods, one of the was the Cambrian explosion. Eventually, the rain takes the CO2 out of the atmosphere and the over all temperature of the Earth cools down. Eventually, the carbon cycle and our continental positions gave us a cooler climate because we have had much less volcanism today than in the distant past. This is due to plate tectonics or continental drift which happens over a very long time, deep time, a geological time scale. Our volcanism will change in the distant future. Widely spead out continents also favor a cooler planetary temperature.

    Mars has a smaller gravity so it will hold onto only the heavier gases with heavier molecular weight. Solar wind stripping is easier because of of Mars low gravity. Mars has an atmospheric composition of mostly CO2, but it is so thin that there is little greenhouse effect. Consequently, the ground on the hottest place on Mars say 50 degrees F at the equator would be below zero a few feet of the ground since thin air cant transport much heat. In a high pressure atmosphere non greenhouse gases also have a small greenhouse effect or contribute to it. Green house gases absorb infra red thermal energy like a greenhouse. The glass in a greenhouse is transparent to sun light or visible light just like our atmosphere. When the sunlight hits the inside of the greenhouse it heats up and that heat is radiated as thermal infra red, the infra red light is trapped by the glass which heats up the air in the greenhouse. The greenhouse effect is independent of the distance of the planet from the star i.e. whether or not it is in the life belt.

    I am not saying there is not any life on TOI 700 d:. There is the strong possibility that it has a thick atmosphere and if it does, then it might not even have any ice caps. It might be a water world, or a warm world with life. Gravity also helps hold an atmosphere since the stronger the gravity, the higher the escape velocity, the higher the temperature and the faster a gas has to be accelerated in order for it to escape and be stripped by the solar wind which might slow down sputtering or atmospheric stripping. Other factors that are including are plate tectonics and volcanism. A large super Earth might not have plate tectonics and there would be no carbon cycle. If the super Earth was an ocean exoplanet, the rain and ocean might take out a lot of CO2. There has to be volcanism because then the planet might cool off without CO2 and the atmosphere has to be replenished. I still think there might be some kind of volcanism so the planet would be a warm one. I think if we get the spectra of these super Earths we will be able know what their geology and climate is.

    • Alex Tolley January 13, 2020, 17:39

      I am not arguing against anything you say here. What I am saying is that the HZ is a theoretical band where a planet of the right size and atmospheric composition could have stable surface water. Whether those conditions are met will determine if the planet is habitable (or inhabited).
      Unless I read your comment incorrectly, you were saying that the orbit of the planet was very close to the inner edge of the HZ. You then appeared to suggest specific planetary conditions for this, which I am saying are irrelevant to the placement of the orbit of teh planet and the boundfaries of the HZ.

  • Geoffrey Hillend January 13, 2020, 23:29

    I apologize if I read more into your post than what was there. We agree that the habitable zone or life belt is where the most likely place to life due to the temperature where water remains liquid, but not frozen or evaporated. The amount of atmosphere does make a difference. A 2.3 Earth mass planet with a thick atmosphere at the outer edge of the life belt will have a warmer temperature than an Earth mass size planet with a thinner atmosphere the same place at the outer edge. The greenhouse effect will make the 2.3 Earth mass planet at the inner edge of the habitable zone warmer than a planet with less atmosphere in the same spot.

    The large mass of TOI 700 d increases the possibility that it might have a considerable atmosphere since it still might take a long time to completely remove an atmosphere through solar wind stripping especially if the super Earth still has some kind of volcanism.

    I will admit that I don’t know what kind of atmosphere TOI 700 d has, but I am only making inferences based on planetary science since a larger mass or larger gravity will retain a larger amount of atmosphere for a longer time not considering geological process on a 2.3 mass planet which are unknown and can be only modeled using physics and geology. I do think astrophysics and scientists will be able to gain a lot of knowledge about a planets geology and climate once we obtain a lot of the spectra of super Earths.

    One thing I left out in the carbon cycle was that the limestone which accumulates at the bottom of the sea from the rain taking the CO2 out of the atmosphere through rivers and rainfall over the sea eventually ends up being forced down into the mantle and melted down and after the plates on the sea floor subduct. These subduction zones are one plate being forced under another down under another into the mantle which is a slow process in deep time. The carbon and oxygen in CaCO3 or limestone(calcium carbonate) is recycled and expelled back into the atmosphere through volcanoes. Bennett, 2008. “Beyond UFOs.”