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

Launched on April 18, 2018, NASA’s Transiting Exoplanet Survey Satellite (TESS) has been systematically surveying about 200,000 of the brightest stars over most of the sky looking for periodic dips in brightness indicating the presence of transiting exoplanets. A follow up to NASA’s highly successful Kepler mission, TESS is approaching the end of its two-year primary mission with its extended mission scheduled to start in July 2020.

Graduate student Emily Gilbert (University of Chicago) was the lead author for the TOI-700 discovery paper submitted in January 2020. (University of Chicago)

While a handful of exoplanet discoveries from TESS have been announced to date, follow up observations of over one thousand exoplanet candidates are now beginning to yield increasing numbers of confirmed finds. During a press conference held on January 6, 2020 at the 235^th meeting of the American Astronomical Society (AAS) in Honolulu, some of the mission’s more interesting new discoveries were revealed. Graduate student Emily Gilbert (University of Chicago) announced the detection of a trio of exoplanets orbiting a red dwarf designated TOI-700 (TESS Object of Interest 700) one of which appears to be a roughly Earth-size world orbiting inside of the Habitable Zone (HZ) of that star. A paper with Gilbert as the lead author, along with a pair of accompanying papers giving more details about this new find, have been submitted for publication in one of the AAS peer-reviewed journals. So what are the prospects that this new find, designated TOI-700d, is 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. Originally misclassified as a Sun-like star in the available version of the TESS Input Catalog (where the star was identified as TIC 150428135), an inspection of the available multi-band photometry by Gilbert et al. instead suggested that KOI 700 was a much smaller M-type star. Gilbert et al. obtained medium resolution spectra of KOI 700 on the night of September 30, 2019 using the Goodman High-Throughput Spectrograph on the Southern Astrophysical Research (SOAR) 4.1-meter telescope located at Cerro Pachon in Chile. Analysis of the resulting spectrum showed that TOI-700 was a spectral type M2V star. Combining the analysis results of this spectrum with existing photometry as well as new photometry obtained August 20, 2019 using the 0.9-meter SMART/CTIO telescope in Chile and a Gaia-derived distance of 101.54±0.07 light years, Gilbert et al. employed validated models of red dwarfs to estimate the properties of KOI-700 summarized in the table below.

Properties of TOI-700 (Gilbert et al.)

As part of their characterization of TOI-700, Gilbert et al. examined 2,500 archived photometric measurements made in the V and g bands over a five year period by the ASAS-SN (All-Sky Automated Survey for SuperNovae) program sponsored by Ohio State University. ASAS-SN uses a network of 20 modest-sized robotic telescopes scattered across the globe primarily to search for supernovae in other galaxies. The team found no evidence of any major flaring by this red dwarf but they did observe regular fluctuations in brightness with an amplitude of 1.26±0.07% and a period of 54.0±0.8 days. This has been interpreted as being the result of starspots rotating into and out of view with the observed period corresponding to the time it takes for this star to rotate. An analysis of 1,137 archived r’-band photometric measurements obtained by the HATSouth (Hungarian Automated Telescope – South) telescope network between February 15 and May 9, 2017 yielded comparable results.

Long-term monitoring of TOI-700 by the ground-based ASAS-SN telescopes revealed a 54-day rotation period. The V-band data are in blue while g-band is in red. Click on image to enlarge. (Gilbert et al.)

This fairly long period of rotation, the lack of major flaring events, the absence of any detectable ultraviolet emissions and its motion through the galaxy all suggest that TOI-700 is a fairly inactive star with an age in excess of 1.5 billion years to a 95% confidence level. It is difficult to pin down the age of this star any more accurately using existing models given its slow evolution over a lifetime that is on the order of a couple of hundred billion years – well over an order of magnitude longer than a larger star like the Sun.

The Search for Exoplanets

Unlike Kepler which stared at a single patch of sky for 3½ years during its primary mission to detect the telltale periodic dips in brightness indicative transiting exoplanets, TESS uses an observing strategy which allows it to search almost the entire celestial sphere. The emphasis in this new survey mission is to observe brighter, nearby stars which are more amenable to follow up observations using existing technology to confirm any exoplanet candidates and characterize their properties.

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

After launch, 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 gravity on TESS’ orbit. From this orbit, 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 are 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 is completed, about 85% of the celestial sphere would have been surveyed with the largest gap 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 has monitored. 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 KOI 700 located only 3° from the south ecliptic pole, the star was observed almost continuously for 11 out of the TESS’ first 13 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 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.)

The TESS Science Processing Operation Center (SPOC) pipeline first identified two transiting exoplanets orbiting TOI-700 in June 2019 based on the first five sectors of observations. A third transiting exoplanet was spotted in August 2019 based on the analysis of the first eight sectors. The periods of these exoplanets, eventually designated TOI-700b, c and d, were 9.98, 16.05 and 37.43 days, respectively. At this point, follow up observations were scheduled to better characterize the star and confirm the planetary nature of the observed dips in brightness as TESS continued its survey of the southern hemisphere.

This plot shows the full 11-sector TESS light curve of TOI-700. Each sector of data is shown with a separate color. Transits of the planets are marked in blue (TOI-700b), pink (TOI-700c), and purple (TOI-700d). TESS observed 25 transits of planet b, 14 transits of planet c, and 8 transits of planet d. Transits that occur during gaps in the TESS data collection do not have a dot below the transit model. Click on image to enlarge. (Gilbert et al.)

Confirming the New Finds

An examination of archival imagery of TOI-700 stretching back to 1982 (since which time TOI-700 had moved 7 arc seconds due to its proper motion) revealed no background stars near the current position of TOI-700 down to a “blue” magnitude ~17. New high resolution images acquired on October 8, 2019 using the Zorro speckle instrument on the 8.1 Gemini South telescope on Mauna Kea and on October 16 using the SOAR speckle imaging camera found no evidence of a previously undetected stellar companion of TOI-700 as close in as 0.54 AU of the star. These observations, along with vetting procedures for the TESS photometric data, effectively eliminate the possibility that a star other than TOI-700 is creating a false positive signal of astrophysical origins further bolstering the transiting exoplanet interpretation for the observed dips in brightness.

On the left are Gemini-South Zorro speckle observations of TOI-700 taken at 832nm and the corresponding contrast curve. The red line fit and blue points in the contrast curve represent the 5-sigma to the background sky level (black points) revealing that no companion star is detected from the diffraction limit (17 mas) out to 1.75 arc sec within a delta-mag of 5 to 8. The reconstructed speckle image (inset) has North up, East to the left and is 2.5 arc sec across. On the right is the SOAR HRCam I-band contrast curve and autocorrelation function (inset). The contrast curve shows that TOI-700 hosts no close companions brighter than delta-I ~5 mag at separations beyond 0.3 arc sec. Clcik on image to enlarge. (Gilbert et al.)

A team of scientists led by Joseph Rodriguez (Center for Astrophysics – Harvard & Smithsonian) requested follow up observations with NASA’s Spitzer Space Telescope to confirm TOI-700d. Spitzer started an 8.9-hour observation session of TOI-700 at 20:19 UT on October 22, 2019 at a wavelength of 4.5 μm using Spitzer’s InfraRed Array Camera (IRAC). A transit of TOI-700d was observed, as predicted using TESS data, eliminating all other possible causes of a false positive signal and confirming the planetary nature of the find. In addition, the 99-day extension of the observation baseline beyond the TESS data set allowed the orbital period of TOI-700d to be refined by 56% compared to the TESS data alone while the superior photometric accuracy of the Spitzer data allowed a 38% improvement of the size measurement of the exoplanet.

These plots show the photometric observations of TOI-700. At top is the TESS light curve with the predicted times of the transits indicated. The three remaining boxes show the light curves for TOI-700b, c and d from all currently available space- and ground-based telescopes. Click on image to enlarge. (Rodriguez et al.)

Rodriguez et al. also secured ground-based observations to attempt to observe a transit of the largest exoplanet, TOI-700c, as part of the TESS Follow-up Observing Program (TFOP). 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 confirming the planetary nature of this find as well. With the observation baseline now extending 108 days after TESS, the orbital period of this world was refined by 30% while the uncertainty in the radius was decreased by 36%.

The properties of the newly discovered exoplanets orbiting TOI-700 are listed in the table below along with any uncertainties where they are important. Also included is the effective stellar flux, S_eff, from 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. & Rodriguez et al.)

Potential Habitability

So, what are the habitability prospects for these new exoplanets? 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 the exoplanets orbiting TOI-700 is to determine what sort of worlds they are: are they rocky planets like the Earth or is it volatile-rich mini-Neptunes 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 three 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 a 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 to indirectly determine the masses of its exoplanets from TESS transit data. Because of the limited number of transits observed over the course of a year, the resulting masses have large uncertainties: 0.42 +2.5/-0.42 M_E (where M_E is Earth mass), 1.1 +5.4/-1.1 M_E and 1.0 +4.1/-1.0 M_E for TOI-700b, c and d, respectively.

A plot of the observed transit time variations (TTVs) for the three exoplanets of the TOI-700 system compared to a series of TTV analysis models. Click on image to enlarge. (Gilbert et al.)

Based on these TTV-derived masses and the radii, all that can be said with certainty is that the largest exoplanet, TOI-700c, has a one-sigma upper limit for its density of 1.9 times that of water suggesting that it is a volatile-rich mini-Neptune with a substantial hydrogen-helium atmosphere. More transit observations over a longer period of time will be required to derive any meaningful density limits using the TTV method for the smaller pair of exoplanets, TOI-700b and d. But at this time, the TTV analysis rules out the presence of other large exoplanets gravitationally interacting with the three currently known worlds.

Without better information on the masses of the transiting planets of TOI-700 immediately available, we are forced to rely on statistical arguments based on the observed mass-radius relationship of other exoplanets whose radii and masses have been measured. A series of analyses of Kepler data and follow-up observations published over the last several years has shown that there are limits on how large a rocky planet can become before it starts to possess increasingly large amounts of water, hydrogen and helium as well as other volatiles making the planet more of a Neptune-like world. An analysis of the mass-radius relationship with a large collection of exoplanetary data by Chen and Kipping suggests that that the gradual transition from rocky to volatile-rich exoplanets starts at about 1.2 R_E again with the probability that a planet is rocky decreasing with increasing radius.

Gilbert et al. used a program called FORECASTER developed by Chen and Kipping to estimate the probable mass range of an exoplanet given its radius based on the population of known exoplanets. Gilbert et al. estimate the masses of TOI-700b, c and d to be 1.07 +0.80/-0.43 M_E, 7.48 +5.89/-3.30 M_E and 1.72 +1.29/-0.63 M_E, respectively. Dynamical simulations using these mass ranges show that the system is stable over long periods of time. While the 6.3 M_E upper limit for the mass of TOI-700c derived from the TTV analysis is towards the lower end of the probable mass range derived using FORECASTER, the likely mass ranges for TOI-700b and d strongly favor a more Earth-like rocky composition as opposed to that of a mini-Neptune.

The Habitable Zone

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 an S_eff value of 0.93 which corresponds to a mean orbital distance of 0.16 AU.

Because of the tight orbits of the three 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. Detailed climate modeling over the last two decades 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 daylight side. But there are limits besides a runaway greenhouse effect to consider when defining the inner edge of the HZ.

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 an 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.

Looking at the trio of exoplanets orbiting TOI-700, TOI 700b and c, with S_eff values of 5.0 and 2.7 times that of Earth, respectively, clearly orbit beyond the inner limits of the HZ for both slow and fast rotating planets. Assuming TOI-700b has retained a substantial atmosphere, it is likely to be a larger and much hotter version of Venus. Given the low density of TOI-700c which precludes an Earth-like composition, it is most likely a very warm mini-Neptune with a dense, hot atmosphere dominated by hydrogen and helium overlying layers of exotic ices which exist only under high temperatures and pressures – the kind of place unlikely to harbor life as we know it. But even if this volatile-rich world has a roughly Earth-size moon, it is unlikely to be habitable and would likely be more Venus-like.

The situation with TOI-700d is much more promising. With a S_eff of 0.86 +0.15/-0.12 times that of the Earth, it comfortably orbits within the HZ of a slow or synchronous rotator (its most likely spin state) and even near the edge of the HZ for a fast rotator even when the uncertainties in the S_eff are taken into account.

The upper panel shows the configuration of the TOI-700 system with the boundaries of the habitable zone (HZ) shown. The lower panel compares the TOI-700 system to five other planetary systems including the Solar System. Click on image to enlarge. (Gilbert et al.)

The third in the trio of papers submitted for publication with the announcement of the discovery of TOI-700d was an examination of 3D climate models for this exoplanet with Gabrielle Suissa (NASA Goddard Space Flight Center) as the lead author. Suissa et al. examined the results a total of 20 self-consistent models assuming ocean-covered and desert conditions on TOI-700d with synchronous and 2:1 spin states (with the latter corresponding to a case where TOI-700d rotates twice for each 37-day orbit). The atmospheres considered had various pressures with compositions similar to those of the modern Earth as well as the assumed condition on the early Earth, an early Earth model with enhanced levels of H₂, a CO₂-dominated early Mars and a simple 1-bar atmosphere of N₂ with no CO₂. The team found global mean surface temperatures that range from 236.7 K to 364.2 K (-36.5° C to +91.1° C) and that even in the coldest case (i.e. N₂ with no CO₂), ~24% of the surface remains ice free near the substellar point. Suissa et al. conclude that TOI-700d “is a robust candidate for a habitable world and can potentially maintain temperate surface conditions under a wide variety of atmospheric compositions.”

The Future

The future prospects for more observations of this system are very bright. While NASA’s Spitzer Space Telescope was retired in January 2020 and cannot provide any further observations, TESS will undoubtedly be making new transit observations during its extended mission starting in July 2020. TOI-700c is large enough to be detected using ground-based instruments further enlarging its transit data base. These enlarged data sets in combination with a better characterization of TOI-700 itself will allow the size of these exoplanets to be refined further as well as their orbits. TTV analysis over this longer baseline should yield better estimates of these worlds’ masses especially TOI-700b and c.

In addition to TTV analysis, precision radial velocity (RV) measurements hold much promise to measure the masses of these exoplanets. According to estimates from Gilbert et al. using the FORECASTER model, TOI-700b, c and d are expected to produce RV variations with semiamplitudes of 0.57, 3.8 and 0.59 meters per second, respectively, give or take 20%. The RV variations produced by TOI-700c are well within the reach of current precision RV instruments in the southern hemisphere such as HARPS and PFS. Unfortunately, the 16-day orbital period of TOI-700c is about one third of the 54-day rotation period of its parent star. Natural stellar “noise” from surface activity modulated by star’s rotation can produce false signals which will complicate the detection of the RV signature of TOI-700c. High cadence observations combined with new data processing techniques to suppress noise from various sources will be required.

The properties of the exoplanets orbiting TOI-700 as a function of incident bolometric flux (equivalent of effective stellar flux) and planet radius. Planets in the lower part of the plot are more likely to be rocky. The color key in the upper right denotes the approximate temperature while the size of the symbols is inversely proportional to the amount of observation time needed to derive the mass using precision radial velocity measurements – bigger planets are easier. Click on image to enlarge. (Rodriguez et al.)

The detection of RV variations produced by the smaller exoplanets, TOI-700b and d, is possible but will be challenging. The only instrument currently available to provide RV measurements of TOI-700 with an accuracy better than 0.5 meters per second is ESPRESSO. Measuring TOI-700b and particularly d will be difficult because of the relatively long orbital period of the latter planet as well as the low expected amplitude and will require excellent instrument stability. Combining precision RV and TTV results might provide a means of overcoming each method’s shortcomings to derive some useful mass measurements.

This plot shows all know transiting exoplanets which orbit in or near optimistic habitable zone (HZ) as a function of the Transmission Spectroscopy Metric – the higher the value, the easier it is to perform transmission spectroscopy during a transit. Click on image to enlarge. (Gilbert et al.)

The KOI-700 system also presents a promising target for future spectroscopic studies to help characterize their atmospheres and compositions. Given its comparatively large size relative to its parent star, TOI-700c is the best target for transmission spectroscopy. Calculations by Suissa et al. using their various model atmospheres for the potentially habitable TOI-700d have shown that NASA’s James Web Space Telescope will be unable to detect its atmosphere, unfortunately. While new, more sensitive instruments will be required to study TOI-700d, it is still among the best potentially habitable exoplanet candidates currently known for such studies, save for the exoplanets orbiting TRAPPIST-1. As scientists continue to comb through the data from the ongoing TESS mission, perhaps even more promising candidates will be found.

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Related Reading

Articles related to NASA’s earlier Kepler mission, which used the transit method to find exoplanets, can be found on this web site’s Kepler mission page.

General References

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”, arXiv 2001.00952 (submitted to AAS Journals), January 3, 2020 [Preprint]

Jeanette Kazmierczak, “NASA Planet Hunter Finds Earth-Size Habitable-Zone World”, NASA Press Release 2020-002, January 6, 2020 [Link]

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”, arXiv 2001.00954 (submitted to AAS Journals), January 3, 2020 [Preprint]

Gabrielle Suissa et al., “The First Habitable Zone Earth-sized Planet from TESS. III: Climate States and Characterization Prospects for TOI-700d”, arXiv 2001.00954 (submitted to AAS Journals), January 3, 2020 [Preprint]