Habitable Planet Reality Check: The Earth-Size Planets of Teegarden’s Star

As engineers and space travel enthusiasts continue to make progress tackling the problems associated with interstellar travel, astronomers around the globe have been busy searching for targets where these interstellar missions could go eventually. On June 18, 2019 an international collaboration of astronomers announced the discovery of two new exoplanets orbiting the nearby Teegarden’s Star which itself had only been found 16 years earlier. Not only are the masses of these new finds similar to Earth’s, but the amount of energy they receive from their host star is also similar opening the possibility that both exoplanets are habitable. So, what are the prospects for the potential habitability of these new exoplanets given what we now know about them and planetary habitability?

Background

Teegarden’s Star is a V magnitude 15.08 ultracool dwarf located in the constellation of Aries near the ecliptic. The star was discovered in 2003 by a team led by Bonnard Teegarden (NASA Goddard Space Flight Center) while they were searching the NEAT (Near Earth Asteroid Tracking) project’s data archive for dim, high proper motion stars. Given the catalog designation of SO J025300.5+165258 (and known by other names like GAT 1370 and 2MASS J02530084+1652532), Teegarden et al. found that the star had a very large proper motion of 5.05 arc seconds per year suggesting that it was especially close. Initial estimates for its distance placed it among the nearest stars known earning it the more easily remembered informal moniker of “Teegarden’s Star”. Current parallax measurements by ESA’s ongoing Gaia mission place Teegarden’s Star at a distance of 12.50±0.01 light years making it the 24^th closest known star system. A summary of the best current values for this star’s key properties are listed in the table below.

Properties of Teegarden’s Star

Overall, the properties of Teegarden’s Star are very similar to those of the notable ultracool dwarf star, TRAPPIST-1 (see “Habitable Plant Reality Check: The Seven Planets of TRAPPIST-1”). But because of its relative closeness, Teegarden’s Star appears from Earth as among the brightest stars of its class making it a prime target for study. Photometry of this ultracool dwarf has revealed no substantial variations in brightness and all its activity indicators point towards it being a magnetically quiet star. The lack of any periodic variation in these measurements implies that whatever photospheric activity Teegarden’s Star displays comes and goes on a time scale shorter than its rotation period suggesting a value in excess of 100 days. This slow rotation, the lack of activity and estimates based on stellar models suggests that Teegarden’s Star is eight or more billion years old – twice the age of the Sun.

Because of its small size and proximity, Teegarden’s Star would be considered a good target for exoplanetary searches. Unfortunately, with this star’s low V magnitude, it cannot be observed by any of the precision radial velocity (RV) surveys operating at visible wavelengths looking for the subtle reflex motion due to an orbiting exoplanet – the technique responsible for most of the exoplanet discoveries among the nearby stars. However, because of its low temperature, this ultracool dwarf, with a J magnitude of 8.39, is significantly brighter at infrared wavelengths for instruments operating at these longer wavelengths.

The Magellan Clay Telescope (right) at the Las Campanas Observatory was used in the Red Optical Planet Survey (ROPS) which made the first radial velocity survey of Teegarden’s Star. (Jan Skowron)

The Red Optical Planet Survey (ROPS) made the first precision RV measurements of Teegarden’s Star in November of 2010 using the MIKE (Magellan Inamori Kyocera Echelle) spectrograph on the 6.5-meter Magellan Clay telescope located at the Las Campanas Observatory in Chile. Because of the spectrograph’s sensitivity out to 0.9 μm in the near infrared and the huge size of the telescope, RV measurements of Teegarden’s Star with an estimated velocity error of about 9.9 meters per second were possible – sufficient to spot super Earth-size exoplanets. In an analysis published in 2012 with J.R. Barnes (University of Hertfordshire – UK) as the lead author, an RMS scatter of 63.3 meters per second was noted in the nine RV measurements acquired over two nights. This suggested that Teegarden’s Star might be orbited by Saturn-size exoplanet in a short period orbit. A dedicated, long-term observation campaign of this star was needed.

The 3.5-meter telescope at the Calar Alto Observatory in Spain used by CARMENES to make precision radial velocity (RV) measurements of Teegarden’s Star. (Centro Astronómico Hispano-Alemán)

An exoplanet survey using an instrument with the required sensitivity was finally available at the beginning of 2016 with the introduction of CARMENES (Calar Alto high-Resolution search for M dwarfs with Exoearths with Near-infrared and optical Échelle Spectrographs). Mounted on the 3.5-meter Zeiss telescope at the Calar Alto Observatory in Spain, CARMENES employs a pair of spectrographs which cover a wavelength range from 0.52 to 1.71 μm where cool stars like Teegarden’s Star are expected to be the brightest. For this work described in a paper with Mathias Zechmeister (University of Göttingen) as the lead author, the international team of astronomers used 238 spectra acquired from January 31, 2016 to March 12, 2019 to derive RVs with a typical instrumental uncertainty of 1.67 meters per second. Complementing these spectrographic data were photometric measurements acquired by seven observatories scattered around the globe. Combined with stellar activity indicators derived from the CARMENES spectra, these data were used to help differentiate between RV variations resulting from a bona fide orbiting exoplanet and spurious signals caused by activity on the surface of Teegarden’s Star (an effect called “jitter”).

The upper panel shows the CARMENES radial velocity measurements of Teegarden’s Star as a function of Julian date used in this analysis. The lower panels show these measurements plotted as a function of phase for planet b (left) and c (right). Click on image to enlarge. (Zechmeister et al.)

The detailed analysis by Zechmeister et al. showed two periodic signals in their data which were consistent with the presence of orbiting exoplanets: one with a period of 4.9100±0.0014 days with a semiamplitude of 2.02 +0.19/-0.20 meters per second and another with a period of 11.409±0.009 days with a semiamplitude of 1.61±0.19 meters per second. A third signal with a period of 25.94 days and a semiamplitude of 0.9 meters per second was also found but, with a false alarm probability about 0.5%, more data will be needed to better characterize this signal and eliminate the possibility that it is a false positive. The larger variation in RV observed by the earlier ROPS survey was not seen in this new data set suggesting that instrument drift was a larger issue than originally estimated in the analysis by Barnes et al. in 2012.

Using the properties of Teegarden’s Star, it is possible to derive the properties of these new exoplanets, designated Teegarden’s Star b and c, which are summarized along with their uncertainties in the table below.

Teegarden’s Star’s Exoplanet Properties

Since the inclination, i, of the orbits of these exoplanets to the plane of the sky cannot be derived directly from RV measurements, only the minimum mass or M_Psini values can be derived with the actual mass, M_P, likely being larger. Still, it is apparent that Teegarden’s Star b and c have masses only somewhat greater than the Earth’s (or M_E). Given the derived uncertainties in the M_Psini values and assuming a random orbit inclination, the one-sigma (or 68% probability) range for possible M_P values for planets b and c are 1.03 to 1.47 M_E and 1.08 to 2.04 M_E, respectively. Also included in the table is the effective stellar flux (S_eff – the total amount of energy received from its host star) of each exoplanet. Once again, these values are similar to Earth’s prompting speculation about the potential habitability of these newly discovered worlds.

Potential Habitability

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 newly discovered exoplanets orbiting Teegarden’s Star is to determine what sort of worlds they are: are they rocky planets like the Earth or volatile-rich mini-Neptunes possessing deep hot atmospheres dominated by hydrogen overlaying layers of exotic high temperature ices with little prospect of being habitable in an Earth-like sense. Unfortunately, the only information currently available about Teegarden’s Star b and c that could help make such an assessment are the M_psini or minimum mass values. The actual masses and the radii of these worlds are needed to calculate their density and help constrain their bulk compositions.

With such tight orbits around their sun, Teegarden’s Star b and c have a probability of 2.1% and 1.2%, respectively, that the plane of their orbits are oriented by random chance to produce transits observable from Earth. Such transits could be used to pin down not only the orbit inclination, i, allowing the actual planet mass to be determined but also the exoplanet’s radius. Zechmeister et al. were able to use their extensive photometric data set to perform an exhaustive search for transits of planet b. With all phases of its 4.91-day orbit thoroughly sampled, there is no evidence for transits of planet b. While the time coverage of the photometric data is insufficient to rule out transits of planet c or other undetected orbiting planets, no transits were detected with a depth greater than about two millimagnitudes – the equivalent of a world about the size of Mars. More data will be needed to perform a thorough search for transiting planets orbiting Teegarden’s Star.

This plot shows all of the photometric data gathered for Teegarden’s Star in support of this analysis as a function of the orbit phase of planet b. Click on image to enlarge. (Zechmeister et al.)

Until we get the data needed to determine the bulk densities of these new finds, statistical arguments can be made about the probability they have a rocky composition. An analysis of the mass-radius relationship for extrasolar planets smaller than Neptune performed by Rogers strongly suggests that the population of known exoplanets transitions from being predominantly rocky planets like the Earth to predominantly volatile-rich worlds like Neptune at radii no greater than 1.6 times that of the Earth or R_E but more likely at 1.5 R_E (see “Habitable Planet Reality Check: Terrestrial Planet Size Limit”). While rocky planets larger than this are possible, they become more uncommon with increasing radius. A planet with a radius of 1.6 R_E and an Earth-like composition would have a mass of about 6 M_E. Even with their currently unconstrained orbit inclinations, the chances that the actual masses of Teegarden’s star b and c exceed this 6-M_E threshold are only around 1.5% and 1.7% respectively.

More recent work by Chen and Kipping with a larger sample of exoplanets suggests that the gradual transition of the exoplanetary population from predominantly rocky planets to volatile-rich worlds starts at about 2 M_E. There is about a 15% chance that Teegarden’s Star B exceeds this threshold while planet c has around a 17% chance. This suggests that these newly discovered exoplanets still have a small chance of being mini-Neptunes. Until a more quantitative estimate can be made based on an analysis of the available data, it seems likely (but not certain) that both these exoplanets are rocky planets with a bulk composition more similar to Earth’s than Neptune’s.

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 outer limit of the HZ 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 an S_eff value of about 0.22 for the outer limit of the HZ of an Earth-sized exoplanet orbiting Teegarden’s Star corresponding to a mean orbital distance of 0.057 AU. The semimajor axes of the orbits of both Teegarden’s Star b and c are comfortably less than this outer limit.

Kopparapu et al. (2013, 2014) conservatively define the inner edge of the HZ 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 its water in a geologically brief time in the process. For an Earth-size planet orbiting Teegarden’s Star, this happens at an S_eff value of 0.86 which corresponds to a mean orbital distance of 0.029 AU. With a S_eff calculated to be 0.37, Teegarden’s Star c orbits well beyond this limit and comfortably within this system’s HZ. Planet b, however, with an S_eff of 1.15, orbits too close to its sun to be considered habitable by this definition. However, there are other definitions for the HZ worth considering for this case.

A comparison of the Sun and Teegarden’s Star as well as its newly discovered exoplanets. The green band shows the approximate limits of the habitable zone (HZ). Click on image to enlarge. (M. Zechmeister)

Because of the tight orbit of Teegarden’s Star b and its age, it would be expected to be a synchronous rotator which keeps the same side pointing towards its sun. Detailed climate modeling over the last two decades shows that synchronous rotation is not the impediment to global habitability as it was once thought. In fact, it has been shown that slow or synchronous rotation can actually result in an increase of the S_eff for the inner edge of the HZ owing to feedback mechanisms which result in the formation of a reflective cloud layer on the daylight side. A study by Kopparapu et al. (2017) which incorporates not only the latest data of how key greenhouse gases transmit and absorb infrared radiation but the effects of short-period synchronous rotation on cloud formation as well suggests that the inner limit of the HZ for Earth-sized synchronous rotators orbiting Teegarden’s Star would be at an S_eff of about 1.13 corresponding to a mean orbital distance of 0.025 AU. Planet b seems to orbit right at this inner limit making it an even bet that it orbits inside the HZ.

While Teegarden’s Star is a relatively quiescent red dwarf today, various forms of elevated activity it would have surely experienced earlier in its life would have had a major impact on the evolution of its planets’ volatile inventories including water. Detailed modelling of the volatile inventories of these worlds validated by observations of these or similar exoplanets will be needed to determine just how “habitable” Teegarden’s Star’s exoplanets are. Given the uncertainties in the various red dwarf HZ models currently available, Zechmeister et al. characterize their new finds as “temperate planets” instead of a “habitable planets”. Still, they do note that their finds have high ESI (Earth Similarity Index) values with planet b having the highest ESI of any currently known exoplanet. Based on the analysis here, Teegarden’s Star c seems to have the better habitability prospects, but planet b is still in the running until more data become available.

With the fast moving Teegarden’s Star currently just 0.3° from the ecliptic plane, it will not be too long before this star moves into the zone where any observers on planet b or c could witness transits of the planets of our solar system across the Sun. Transits of the Earth are expected to start in 2044 followed by transits of Mars starting in 2190. For the next two centuries, any observers in the Teegarden’s Star system will be able to observe transits of Mars, Earth and Mercury allowing the characterization of these planets much as we are now doing with distant transiting exoplanets. Transits of Mars will stop in 2438 with Earth’s transits disappearing about 58 years later.

This sky map shows the motion of Teegarden’s Star through the transit zones of our solar system’s planets over the next two millennia. Click on image to enlarge. (Zechmeister et al.)

Besides being an interesting coincidence, these transits present an opportunity for SETI. The reasoning goes that when the Earth enters the transit zone of a distant star, its presence and basic characteristics could be assessed by any alien scientists that might be present using technology no more advanced than what we possess today. Now alerted to the presence of the Earth (and its potential habitability), this civilization could start transmitting a signal in hopes of contacting us. With transits of the Earth across the face of the Sun as seen from Teegarden’s Star beginning in 2044, we could conceivably start receiving a signal from any neighbors there not long afterwards. We could even start sending our own signals to get their attention starting in 2032 (taking into account light travel time to Teegarden’s Star) and expect a response to our signal as early as 2056.

Summary

The discovery of Teegarden’s Star b and c presents scientists with yet another opportunity to study nearby “temperate planets” in hopes of more fully characterizing their actual properties. Based on what we known today, it appears likely that both of these new finds are rocky worlds like the Earth with much smaller probability that they are mini-Neptunes. The prospects for the potential habitability of Teegarden’s Star b do not appear to be as good as those of planet c owing to former’s close proximity to the inner limit of the habitable zone as well as being more exposed to any enhanced activity earlier in the evolution of Teegarden’s Star – this despite the record setting Earth Similarity Index (ESI) value for planet b. Still, these new finds do seem to have reasonable prospects of being habitable given the gaps in our understanding of planetary habitability especially for ultracool dwarfs like Teegarden’s Star. Future study of these and similar exoplanets should help provide greater insights in this question. And with hints of a third exoplanet in this system (not to mention the possibility of still others), Teegarden’s Star could be as an important laboratory for studying the evolution of exoplanets orbiting ultracool dwarfs as the better known TRAPPIST-1.

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Related Reading

For a complete collection of articles about our other neighboring star systems and the searches for exoplanets orbiting them, see Drew Ex Machina’s page on Nearby Stars.

General References

J. R. Barnes et al., “Red Optical Planet Survey: a new search for habitable earths in the southern sky”, Monthly Notices of the Royal Astronomical Society, Vol. 424, No. 1, pp 591-604, July 2012

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

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

Leslie A. Rogers, “Most 1.6 Earth-Radius Planets are not Rocky”, The Astrophysical Journal, Vol. 801, No. 1, Article id. 41, March 2015

B. J. Teegarden et al., “Discovery of a new nearby star”, The Astrophysical Journal Letters, Vol. 589, No. 1, pp L51-L53, May 20, 2003

M. Zechmeister et al., “The CARMENES search for exoplanets around M dwarfs: two temperate Earth-mass planet candidates around Teegarden’s Star”, accepted by Astronomy & Astrophysics, June 12, 2019