Habitable Planet Reality Check: TOI-700e Discovered by NASA’s TESS Mission

During a press conference held on January 6, 2020 at the 235^th meeting of the American Astronomical Society (AAS) in Honolulu, Emily Gilbert (then a graduate student at the University of Chicago) announced the discovery of three exoplanets orbiting a red dwarf star being monitored by NASA’s Transiting Exoplanet Survey Satellite (TESS) which had been launched two years earlier. Dubbed TOI-700 (TESS Object of Interest 700), one of the three exoplanets detected, called TOI-700d, proved to be an Earth-size world orbiting comfortably inside the habitable zone (HZ) of this dim sun (see “Habitable Planet Reality Check: TOI-700d Discovered by NASA’s TESS Mission”). This was the first potentially habitable exoplanet discovery made using TESS.

An artist’s rendering of NASA’s TESS spacecraft. (NASA)

Since this time, Gilbert and her collaborators have continued to observe TOI-700 to refine our knowledge of this star and its multi-planet system. On January 10, 2023 at the 241^st meeting of the AAS in Seattle, Dr. Gilbert (now a post-doctoral fellow at the Jet Propulsion Laboratory) announced the results of their new analysis which included another year’s worth of data gathered during TESS’ extended mission as well as ground-based observations. In addition to refining our knowledge of TOI-700, Gilbert et al. announced the discovery of a fourth exoplanet in this system – another Earth-sized exoplanet which orbits inside the system’s optimistic HZ. So, what do we know about this new exoplanet and the claim that it might be potentially habitable?

Background

The star TOI-700 (also known by the designation 2MASS J06282325-6534456) is a V magnitude 13.1 star located near the south ecliptic pole in the constellation of Dorado – the Swordfish. Ground observations performed in 2019 during the efforts to validate the original discovery and the latest analysis shows TOI-700 to be a M2.5V red dwarf star at a distance of 101.50±0.09 light years. The properties of TOI-700 are summarized in the table below:

Properties of TOI-700 (Gilbert et al. 2023)

The primary instrument used to search for and observe the exoplanets found orbiting TOI-700 has been NASA’s TESS which monitors the brightness of stars looking for telltale dimmings caused by a transiting exoplanet. After launch on April 18, 2018, TESS was placed into an elongated 108,000 by 375,000-kilometer geocentric orbit with an inclination of 37°. With an orbital period of 13.7 days, the orbit is in a 2:1 resonance with the Moon and is oriented to even out the effects of our natural satellite’s gravitational effects on TESS’ orbit. From this perch in cislunar space, TESS uses an array of four CCD cameras with a total field of view of 24° by 96° to stare at a strip of sky stretching from an ecliptic pole to nearly the ecliptic itself. After observing a strip of sky, known as a “sector”, for 27.4 days (i.e. two TESS orbits around the Earth), the gaze of TESS’ telescope is moved to an adjacent sector to continue its survey. After 13 sectors covering almost half of the sky centered on the south ecliptic pole were observed over the course of a year, the survey continued for the next year centered on the north ecliptic pole. After its two-year primary mission was completed on July 4, 2020, about 200,000 stars over 75% of the celestial sphere had been surveyed with the largest gap in coverage being along the ecliptic itself.

A diagram of the TESS observation sectors shown in ecliptic coordinates. The color-coded sphere on the right shows the number of days each part of the sky TESS can monitor. Click on image to enlarge. (NASA/GSFC)

With this approach, most of the stars in the sky are only observed for 27.4 days by TESS allowing the repeated detection of transiting exoplanets in fairly short-period orbits of a couple of weeks or less, unlike Kepler which could detect exoplanets with orbital periods of up to a year or so. However, near the ecliptic poles where TESS’ sectors overlap, stars can be observed for much longer stretches of time. In the case of TOI-700 located only 3° from the south ecliptic pole, the star was observed almost continuously during the mission’s Year 1 for 11 out of the TESS’ first 13 primary-mission sectors covering the southern hemisphere from July 25, 2018 to July 18, 2019. TOI-700 was not observed as part of Sectors 2 and 12 because the image of the star unluckily fell into gaps between the arrays of CCDs. This nearly continuous photometric record allowed the detection of multiple transits of any exoplanets with orbital periods less than about four months or so.

This is a mosaic of the first 13 sectors observed by TESS during Year 1 centered on the southern ecliptic pole. The part of the sky enclosed by the dashed blue circle represents the continuously observed zone. The pair of successive closeup images indicates the position of TOI-700. Click on image to enlarge. (Gilbert et al. 2020)

As part of its extended mission, TESS started by reobserving the sectors anchored to the south ecliptic pole for its Year 3 campaign. TOI-700 was observed during 10 out of the first 12 southern hemisphere sectors of the extended mission between July 5, 2020 and May 26, 2021 (with TOI-700 not being observed in the missing sectors because the image of the star fell into gaps between the arrays of CCDs, once again). During 10 sectors of these Year 3 observations, TESS monitored the brightness of TOI-700 with a 20-second cadence instead of the 2-minute cadence used during Year 1. This faster rate of data collection, among other things, allowed for a search for short duration flares on TOI-700. Only four events were detected during the nearly nine months of monitoring which later analysis attributed to cosmic rays strikes on the CCD. Combined with the lack of any white-light flare events of longer duration detected in TESS’ 2-minute cadence data during Year 1 and 3, TOI-700 appears to be a fairly inactive red dwarf. This lack of stellar activity and the 54-day period of rotation (derived from archived ground-based photometric measurements) means that TOI-700 has an age in excess of 1.5 billion years. Current models of stellar evolution for red dwarfs cannot be more precise given the slow evolution of this type of star with an expected lifetime on the order of a couple of hundred billion years.

New Observations & A New Discovery

With nearly two full years of photometric data now available from TESS for TOI-700, the mission’s Science Processing Operation Center (SPOC) data processing pipeline readily identified the original three exoplanets whose detection was announced in 2020. But in addition to these three worlds, a multi-sector search performed on July 31, 2021 also found a much more subtle “Threshold Crossing Event” or TCE. A total of seven events were found in the Year 1 data with another seven in the Year 3 data (although one of these events coincided with the transit of one of the other known exoplanets in this system). It was because of these extra events observed in Year 3 that they were confidently identified through the noise in the data. Additional analysis and vetting of the observations eliminated a range of false alarm scenarios resulting in the upgrading of the TCE on November 21, 2021 to a TOI (TESS Object of Interest) – a potential fourth exoplanet, cataloged as TOI-700e, had been detected.

Here are the light curves for the four TOI-700 planets phase folded to each of their orbital periods. The gray dots are the TESS 2-minute cadence data, the pink dots are the data binned to 10-minute cadence, and the purple lines show the best fit of the data. Click on image to enlarge. (Gilbert et al. 2023) transit

In order to eliminate the possibility that a distant eclipsing binary, whose image was blended with that of TOI-700 in the TESS data, was responsible for the observed dips in brightness, additional observations from ground-based telescopes were needed. During a predicted transit of TOI-700e on December 27, 2021, TOI-700 and its vicinity were observed using the 61-cm CAO (Campocatino Austral Observatory) telescope located at El Sauce Observatory in Chile. A series of 6-minute, clear filter images effectively eliminated the possibility that 66 out of the 86 stars seen withing 2.5 arc minutes of the position of TOI-700 were eclipsing binaries. The remaining stars were simply too dim to explain the decrease in apparent brightness of TOI-700 attributed to TOI-700e. Additional vetting procedures has placed the probability that TOI-700e is a false positive at 2.74X10^-4 strongly favoring the interpretation that TOI-700e is an exoplanet locked in an orbit with a period of 27.8-day orbit around TOI-700.

The next step was not only to characterize the properties of TOI-700e, but also refine our knowledge of the three previously discovered exoplanets in this system. In addition to the Year 1 and 3 photometric data, Gilbert et al. also used photometric data from other sources as well. TOI-700 was observed on November 1, 2019 using the one-meter Las Cumbres Observatory Global Telescope Network (LCOGT) telescopes located at the South African Astronomical Observatory (SAAO). LCOGT easily detected the predicted transit of TOI-700c, which is the largest transiting exoplanet known in this system. Before it ended operations on January 30, 2020, NASA’s Spitzer Space Telescope used its InfraRed Array Camera (IRAC) to observe transits of TOI-700d at a wavelength of 4.5 μm on October 22, 2019 and January 5, 2020. The quality of the Spitzer photometric data is far superior to that of TESS allowing a more accurate determination of this potentially habitable exoplanet’s properties.

The properties of the newly discovered TOI-700e and updated data for the other three exoplanets previously found orbiting TOI-700 are listed in the table below along with any uncertainties, where they are important. The data are ordered by increasing distance from TOI-700 and not alphabetically (which corresponds to the order they were discovered). Also included is the effective stellar flux, S_eff, from the latest work by Gilbert et al. which provides a measure of the amount of energy each planet receives from its sun compared to the Earth.

TOI-700 Exoplanet Properties (Gilbert et al. 2023)

Potential Habitability

So, what are the habitability prospects for TOI-700e? A thorough assessment of the habitability of any extrasolar planet would require a lot of detailed data on the properties of that planet, its atmosphere, its spin state, the evolution of its volatile content and so on. Unfortunately, at this very early stage, the only information typically available to scientists about extrasolar planets are basic orbit parameters, a rough measure of its size and/or mass and some important properties of its sun. Combined with theoretical extrapolations of the factors that have kept the Earth habitable over billions of years (not to mention why our neighbors are not habitable today), the best we can hope to do at this time is to compare the known properties of extrasolar planets to our current understanding of planetary habitability to determine if an extrasolar planet is “potentially habitable”. And by “habitable”, I mean in an Earth-like sense where the surface conditions allow for the existence of liquid water – one of the presumed prerequisites for the development of life as we know it. While there may be other worlds that might possess environments that could support life, these would not be Earth-like habitable worlds of the sort being considered here.

The first step in assessing the potential habitability of TOI-700e is to determine what sort of world it is: is it a rocky planet like the Earth or is it volatile-rich mini-Neptune with little prospect of being habitable in an Earth-like sense. If we know the radius and mass of an exoplanet, its mean density can be readily calculated which in turn can be used to constrain its bulk composition. While the radii of the exoplanets orbiting TOI-700 have been derived from transit measurements, their masses are another matter.

Considering the tight nature of this planetary system, the four exoplanets of TOI-700 will gravitationally interact with each other causing the observed timing of their transits to vary over long periods. This is especially true with TOI-700b and c which are near an 8:5 orbital resonance which tends to amplify the magnitude of the variations. Gilbert et al. performed a Transit Timing Variation (TTV) analysis of the TOI-700 system in order to attempt to determine the masses of its exoplanets indirectly from the transit data. Unfortunately, the analysis of the data available to date shows that the TTVs are too subtle to yield any usable mass values. However, based on a statistical analysis by Chen & Kipping of the mass-radius relationship for exoplanets with known radii and masses, the estimated mass of TOI-700e is about 0.85 +0.67/-0.34 times that of the Earth with an 87% probability that it is a rocky planet like its potentially habitable neighbor, TOI-700d.

Another important criterion which can be used to determine if a planet is potentially habitable is the amount of energy it receives from its parent star known as the effective stellar flux or S_eff. According to the work by Kopparapu et al. (2013, 2014) on the limits of the habitable zone (HZ) based on detailed climate and geophysical modeling, the inner limit of the HZ is conservatively defined by the runaway greenhouse limit where a planet’s temperature would soar even with no CO₂ present in its atmosphere resulting in the loss of all of its water in a geologically brief time in the process. For an Earth-mass planet orbiting TOI-700, this happens at a S_eff value of 0.93 which corresponds to a mean orbital distance of 0.16 AU.

The outer limit of the HZ, based on the work by Kopparapu et al. (2013, 2014), is conservatively defined by the maximum greenhouse limit beyond which a CO₂-dominated greenhouse is incapable of maintaining a planet’s surface temperature. Instead of helping to heat the atmosphere, the addition of more CO₂ beyond this point makes the atmosphere more opaque causing the surface temperatures to drop instead of increase. Kopparapu et al. (2013, 2014) suggests a S_eff value of about 0.22 for the outer limit of the HZ of an Earth-sized exoplanet orbiting TOI-700 corresponding to a mean orbital distance of 0.32 AU. With this conservative definition of the HZ, TOI-700d, with a S_eff of 0.85 times that of Earth orbits comfortably inside the HZ, as originally claimed three years ago (see “Habitable Planet Reality Check: TOI-700d Discovered by NASA’s TESS Mission” for a more detailed discussion). On the other hand, TOI-700e with a S_eff of 1.27 orbits too closely to its sun to meet this conservative definition of the HZ. However, there are other definitions of the inner edge of the HZ  worth considering.

Because of the tight orbits of the four exoplanets orbiting TOI-700, they would be expected to be synchronous rotators which keep the same side pointing towards their sun during the course of their orbits. Increasingly detailed climate modeling over the last quarter century has shown that synchronous rotation is not the impediment to global habitability as it was once thought. In fact, it has been predicted that slow or synchronous rotation can actually result in an increase of the S_eff corresponding to the inner edge of the HZ owing to feedback mechanisms which result in the formation of a reflective cloud layer on the perpetually daylit side of the planet.

A diagram showing the planets orbiting TOI-700 with the conservative habitable zone (HZ) shown in green and a more optimistic definition of the limits of the HZ in light green. Click on image to enlarge. (NASA/JPL-Caltech/Gilbert)

A more recent paper by Kopparapu et al. (2017), which incorporates the latest data of how key greenhouse gases transmit and absorb infrared radiation, suggests that changes in the atmospheric structure for synchronous rotators can lead to rapid and permanent water loss for an Earth-size exoplanet at a S_eff of around 1.34 even before a runaway greenhouse effect sets in. With the permanent loss of water, the carbonate-silicate cycle, which helps act as a global thermostat breaks down, allowing CO₂ to build up in the atmosphere resulting in a dry runaway greenhouse much as Venus experiences today in our own solar system. By this more optimistic definition of the inner edge of the HZ corresponding to a distance of 0.13 AU, TOI-700e would seem to be a candidate for being potentially habitable as claimed by Gilbert et al. who used a different definition for an “optimistic” HZ.

But even if TOI-700e proves to be more like Venus than Earth, important insights can be gleaned about Earth-size worlds in general. While Earth and and is near-twin, Venus, have S_eff values of 1.00 and 1.91, respectively, the similar-sized TOI-700d and e have slightly different S_eff values of 0.85 and 1.27. Gilbert et al. point out that comparing the properties of these exoplanets spanning a slightly different span of S_eff values could shed important insights into the divergent evolutions of Venus and Earth as well as similar size exoplanets elsewhere.

The Future

A comparatively bright star with multiple, small transiting planets (including a pair of potentially habitable ones) is a tempting target for future investigation. As part of its continuing extended mission, TESS began to observe the southern hemisphere in January 2023 as part of its Year 5 campaign and is expected to observe TOI-700 for 9 sectors. This will provide not only more data on the currently known transiting exoplanets in this system over a longer time baseline, but will also allow for the search of still smaller transiting planets potentially lurking in this system, as well as continue to monitor TOI-700 for flare activity.

Already in the works by Dr. Gilbert and her collaborators is a paper being prepared about 15 orbits worth of data on TOI-700 acquired by NASA’s Hubble Space Telescope. Also in the pipeline is an analysis of 91 spectra acquired by ESPRESSO (Echelle Spectrograph for Rocky Exoplanet- and Stable Spectroscopic Observations) mounted on the European Southern Observatory’s Very Large Telescope (VLT) located on Cerro Paranal in the Atacama Desert of northern Chile. An analysis of the precision radial velocity measurements from ESPRESSO, in combination with photometric data, could be used to help pin down the masses of the known planets in this system as well as reveal additional exoplanets. Unfortunately, predictions made by Suissa et al. show that NASA’s James Webb Space Telescope (JWST) does not have the required performance to probe the atmospheres of the planets orbiting TOI-700 – this task will be left for the next generation of instruments. Still, the continued study of TOI-700 promises to reveal much about smaller, Earth-size exoplanets in our neighborhood.

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Related Video

Here is a brief video produced by NASA Goddard Space Flight Center about TOI-700e.

Related Reading

“Habitable Planet Reality Check: TOI-700d Discovered by NASA’s TESS Mission”, Drew Ex Machina, March 11, 2020 [Post]

General References

“Second Earth-sized World Found in System’s Habitable Zone”, NASA Exoplanet Exploration Website, January 10, 2023 [Post]

Jingjing Chen and David Kipping, “Probabilistic Forecasting of the Masses and Radii of Other Worlds”, The Astrophysical Journal, Vol. 834, No. 1, Article id. 17, January 2017

Emily A. Gilbert et al., “The First Habitable Zone Earth-sized Planet from TESS. I: Validation of the TOI-700 System”, The Astronomical Journal, Vol. 160, no. 3, Article ID 116, September 2020

Emily A. Gilbert et al., “A Second Earth-Sized Planet in the Habitable Zone of the M Dwarf, TOI-700”, arXiv:2301.03617 (submitted to Astrophysical Journal Letters), January 9, 2023 [Preprint]

R. K. Kopparapu et al., “Habitable zones around main-sequence stars: new estimates”, The Astrophysical Journal, Vol. 765, No. 2, Article ID 131, March 10, 2013

Ravi Kumar Kopparapu et al., “Habitable zones around main-sequence stars: dependence on planetary mass”, The Astrophysical Journal Letters, Vol. 787, No. 2, Article ID. L29, June 1, 2014

Ravi Kumar Kopparapu et al., “Habitable Moist Atmospheres on Terrestrial Planets near the Inner Edge of the Habitable Zone around M Dwarfs”, The Astrophysical Journal, Vol. 845, No. 1, Article ID. 5, August 2017

Joseph E. Rodriguez et al., “The First Habitable Zone Earth-Sized Planet from TESS II: Spitzer Confirms TOI-700d”, The Astronomical Journal, Vol. 160, No. 3, Article ID 117, September 2020

Gabrielle Suissa et al., “The First Habitable Zone Earth-sized Planet from TESS. III: Climate States and Characterization Prospects for TOI-700d”, The Astronomical Journal, Vol. 160, No. 3, Article ID 118, September 2020