Past, Present and Future Stars That Can See Earth As a Transiting Exoplanet

PAST, PRESENT AND FUTURE STARS THAT CAN SEE EARTH AS A TRANSITING EXOPLANET

L. Kaltenegger1,2∗, J. K. Faherty3 1Carl Sagan Institute, Cornell University, Space Science Institute 302, 14850 Ithaca, NY, USA, 2Astronomy Department, Cornell University, Space Science Institute 302, 14850 Ithaca, NY, USA 3Department of Astrophysics, American Museum of Natural History, Central Park West 79thst., NY 10024, USA *email: [email protected]

In the search for life in the cosmos, transiting exoplanets are currently our best targets. In the search for life in the cosmos, transiting exoplanets are currently our best targets. With thousands already detected, our search is entering a new era of discovery with upcoming large telescopes that will look for signs of life in the atmospheres of transiting worlds. However, the universe is dynamic, and which stars in the solar neighborhood have a vantage point to see Earth as a transiting planet1–4 and can identify its vibrant biosphere since early human civilizations are unknown. Here we show that 1,715 stars within 326 light-years are in the right position to have spotted life on a transiting Earth since early human civilization, with an additional 319 stars entering this special vantage point in the next 5,000 years. Among the stars are 7 known exoplanet hosts that hold the vantage point to see Earth transit, including Ross-128, which saw Earth transit in the past, Teegarden's Star, and Trappist-1, which will start to see Earth transit in 29 and 1,642 years, respectively. We found that human-made radio waves have swept over 75 of the closest stars on our list already.

To obtain the dynamic picture of which estimate stellar spectral types and find that M stars can see the Earth transit1–4 and how many dwarfs dominate our sample. That agrees with years such a viewpoint holds, we evaluated the our understanding of the initial mass- kinematics propagated through time using distribution10 for the Milky Way disk ESA's Gaia Mission early Data Release 3 population. The 2,034 objects contain 194 G (eDR3)5. stars like our Sun and 12A, 2B, 87F, 102K, We identify 2,034 1,520M stars, 7L, 1T, and 109 white dwarfs (https://github.com/jfaherty17/ETZ) stars in (WD) (see Table 1). At least 12 stars are also the Gaia Catalog of Nearby Stars (GCNS)6 that on the giant branch. There are numerous well- are in the Earth transit zone (ETZ)3 over +/- studied stars in this list (e.g., Wolf 359, Ross 5,000years: 313 objects were in the ETZ in the 128, Teegarden's star) as well as previously past, 319 will enter the ETZ in the future, and unknown nearby sources identified in Gaia 1,402 have been in the ETZ for some time. eDR3 (e.g., Gaia EDR3 Given that Earth began transmitting radio 4116504399886241792). waves into the solar neighborhood about 1,402 sources can currently see Earth 100years ago7, we also identify the sub-sample transit, including 128 G stars like our Sun, 10A, of 75 stars located in a 30pc sphere that Earth's 1B, 63F, 73K, 1050M stars, 2L, and 75WDs. radio signals have washed over. SETI observations of stars in the ETZ have We find a large diversity among the 2,034 recently been started11,12. objects that enter/exit the ETZ over Isotopic data indicate that life on Earth +/5,000years. Using the Gaia colors and started by about 3.8 to 3.5 Gya13,14, which 8,9 absolute magnitude, MG (Figure 1), we coincides with the end of the heavy

1 bombardment phase15. While we cannot infer temperate, habitable zone (HZ). This system the time needed for life to start on any will enter the ETZ in 1,642years and remain for exoplanet from Earth's history, the early signs 2,371years. for life on Earth are encouraging. Our ETZ Three more exoplanet systems beyond catalog includes all potentially habitable world 30pc but within 100pc have been in the ETZ for host stars regardless of mass or evolved state thousands of years: K2-240 at 73.043pc hosts given the uncertainty of how/when/where life two transiting planets. The system entered the might arise and survive16–20. ETZ more than 5,000years ago and will remain Of the 2,034 objects in the ETZ, 67% in the ETZ well past 5,000years into the future. (1,355) are also in the restricted ETZ (rETZ)3 - K2-65 at 63.104pc, with one transiting planet, which guarantees a minimum view of Earth's entered the ETZ 2,183years ago and will stay transit for 10 hours (see Table 1): 311 were in in the ETZ past 5,000years in the future. K2- the rETZ in the past, 330 will enter the rETZ in 155 at 72.932pc hosts three transiting planets. the future, and 713 have been in the rETZ in the The system entered the ETZ more than +/-5,000-year time window. This sample is 5,000years ago and will remain for another mainly composed of cool stars, with colors and 3,118years. magnitudes consistent with 6A, 57F, 121G, Estimates of the number of rocky planets in 72K, 1,021M stars, 5L, 1T, and 72WDs. the HZ22 in the literature are given for various Out of the 2,034 objects in the ETZ, 117 values of a planet's radius limit and HZ objects lie within 30pc (~ 100 light-years) of limits23,24. New estimates place it from the Sun. Among those sources, 29 were in the 0.58+0.73/-0.33 to 0.88+1.28/-0.51 planets per ETZ in the past, 42 will enter it in the future, star24 for the empirical HZ. Then, the Empirical and 46 have been in the ETZ for some time. HZ limits are set by the flux a young Mars and These 46 objects would be able to see Earth a young Venus received when there is no more transit while also being able to detect emitted evidence for liquid water on their surfaces22. radio waves from Earth7, which would have The inner limit is not well known because of reached those stars by now: 2F, 3G, 2K, 34M the lack of reliable geological surface history stars, and 5WDs (see Table 1). for Venus beyond 1 billion years ago. Seven of the 2,034 stars are known While the discussion on the occurrence rate exoplanet host stars: table 2 shows them sorted of rocky planets is ongoing, here, we use a by distance from the Sun. Four of the planet- pessimistic rate of 25% to estimate that there hosts are located within 30pc (~100 light- are potentially 508 rocky worlds in the HZ of years). our full sample. Restricting the selection to the The Ross128 system at 3.375pc is the 13th distance radio waves from Earth have traveled- closest system to the Sun and the second closest about 100 light-years - leads to an estimated 29 system with a transiting Earth-size exoplanet. potentially habitable worlds that could have Ross128 would have seen Earth transit for seen Earth transit and also detect radio waves 2,158years – from 3,057 until 900years ago. from our planet. Teegarden's star system, the 25th closest system We found that 43% (868) of stars in our to the Sun at 3.832pc, hosts two non-transiting sample spend at least 10,000years, 68% (1,380) Earth-mass worlds. It will enter the ETZ in at least 5,000years, and 94% (1,910) at least 29years21 for 410years. The GJ9066 system at 1,000years in the ETZ. Sources closer to the 4.470pc hosts one non-transiting planet, will Sun have larger proper motion values than enter the ETZ in 846years and remain in it for those further away, thus stars in our closest 932years. The Trappist-1 system at 12.467pc neighborhood move in and out of the ETZ hosts 7 Earth-size planets, with 4 of them in the faster than more distant objects. The average

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Figure 1: Stars that can see Earth transit since early human civilization: The color-magnitude diagram of the Gaia Catalogue of Nearby Stars (black) limited to sources with RUWE<1.4, photometric signal to noise (in Gaia G and GRP)>100, and parallax uncertainties better than 5%. Overplotted are the 2,034 sources that cross the ETZ in the time interval of +/-5,000years (dark red five-point stars have RUWE<1.4, grey filled circles RUWE>1.4). MG is Gaia magnitude. Data at: https://github.com/jfaherty17/ETZ.

Table 1: Sample of the full machine-readable table of ETZ stars sorted by distance 1 Star Name Distance Earth Transit Zone (ETZ) Name GAIA eDR3 [lightyears] Enter Exit Total When [yr] [yr] [yr] Wolf 359 3864972938605115520 7.9 -46 432 480 Past to future Ross 128 3796072592206250624 11 -3057 -900 2158 Past Teegardens star 35227046884571776 12.5 29 438 410 Future

Table 2: Sample of the full machine-readable table of exoplanet hosts in the ETZ sorted by distance Star Name Distance Earth Transit Zone (ETZ) Exoplanet Host GAIA eDR3 [light Enter Exit Total When years] [yr] [yr] [yr] Ross 128 3796072592206250624 11 -3057 -900 2158 Past Teegarden's Star 35227046884571776 12.5 29 438 410 Future GJ 9066 76868614540049408 846 14.6 846 1777 932 Future TRAPPIST-1 2635476908753563008 40.6 1642 4012 2371 Future K2-65 2613211076737129856 205.8 -2183 5000 7184 Past to future K2-155 145333927996558976 237.9 -5000 3118 8119 Past to future K2-240 6257625719430982016 238.2 -5000 5000 10000 Past to future

1 full table in Supplement Material and at https://github.com/jfaherty17/ETZ

2 Methods The European Space Agency's Gaia unit weight error (RUWE) value from the mission has revolutionized our understanding catalog to flag potentially poor candidates. As of the local solar neighborhood. Gaia released recommended6, a RUWE near unity is the over 1.3 billion sources with 5-parameter signature of a robust astrometric solution. astrometric solutions in data release 2 (proper Values larger than 1.4 are considered suspect motion and parallax values)8. The Gaia for various reasons, including unresolved catalog's early DR3 in Dec 20205 contained the binarity, variability, and crowding. Gaia Catalogue of Nearby Stars (GCNS)6. The Among our full sample of 2,034 objects GCNS is a self-defined "clean" and well- from eDR3 characterized collection of objects within (https://github.com/jfaherty17/ETZ), which 100pc of the Sun, including at least 92% of are in the ETZ for the +/-5000-year period, 349 stars of stellar types down to M96. objects have a RUWE >1.4. Like the procedure We converted right ascension and used to discover the nearby L dwarf WISE declination values to ecliptic latitude and J192512.78+070038.840 which was in the longitude values for the total 100 pc GCNS Galactic plane, we visually inspected each sample to determine which stars could see source using the citizen scientist developed Earth transit from their position. The Earth image viewing tool called Wiseview41 to transit zone (ETZ)3 corresponds to a thin strip ensure that crowding did not insert a false around the ecliptic as projected onto the sky detection. with an ecliptic latitude width of 0.528 degrees. We also examined the Gaia color- The smaller restricted Earth Transit zone magnitude diagram (CMD) for all targets to (rETZ)3 with an ecliptic latitude width of 0.264 flag any that appeared in suspect areas. Figure degrees denotes the region from which the 1 shows the CMD for the GCNS catalog Earth's transit can be seen for a minimum of 10 (black) - for (G-GRP) colors – limited to sources hours. with RUWE<1.4, photometric signal to noise In the 302,197 stars in the 100pc GCNS (in Gaia G and GRP)>100, and parallax sample, we identify 1,402 objects currently uncertainties better than 5%. Overplotted are located at ecliptic latitude between +/-0.264 the 2,034 sources that cross the ETZ in the time degrees. We compared our sample with an interval of +/-5,000years (dark red five-point earlier study4 that found 1,004 main sequence stars have RUWE<1.4, grey filled circles have stars in the ETZ using the Transiting Exoplanet RUWE>1.4). Data are available at Survey Satellite (TESS)38 Input Catalog https://github.com/jfaherty17/ETZ. We also 39 8 (TIC) matched to Gaia DR2 . We find that in examined the Gaia (GBP-GRP) CMD to the updated eDR3 Gaia catalog, 21 of the complement the analysis. Both Gaia CMD's original objects are now either outside of 100 show scatter across the stars, which is a pc (20) or are unreported in Gaia eDR3 (1). function of several parameters such as varying Thus, our sample has 983 sources in common stellar temperatures, binarity, and age9,42,43. with the earlier sample of stars that can see Note that the (G-GRP) color is discerning for the Earth transit now. Note that the earlier 1004 lowest temperature sources (e.g., M dwarfs), 4 stars sample excluded evolved stars from their while the (GBP-GRP) color is helpful for a close selection and limited the Gaia quality flag in examination of the higher mass sources. We the TIC to unity. primarily used the (G-GRP) CMD to determine The reliability of the astrometry is critical a spectral type (SpT) estimate of each source; to determining the robustness of a candidate in however, we turned to the (GBP-GRP) color for the ETZ. As such, we use the re-normalized

3 confirmation in the case of higher mass stars position in the ETZ. Therefore, radial velocity and white dwarfs. has a minimal effect on the analysis. However, The vast majority of our ETZ sources lie to truly understand the volume of sources within the full GCNS sample's scatter on the analyzed, one would want all astrometric CMD. However, a small number of sources components. A dedicated RV campaign for show photometry or astrometry that is likely these objects will be necessary to accomplish suspect even for objects with RUWE<1.4, that. given the significant number of objects with To compile a list of past and future host Gaia (G-GRP)> 1.5. That might be a sign of a stars in the ETZ and how long they spent with given object's intrinsic properties (like age, a view of Earth transiting, we converted the metallicity, binarity). Table 1 lists parameters GCNS positions into XYZ cartesian for the full 2,034 ETZ sample (data at coordinates using subroutines from the Banyan https://github.com/jfaherty17/ETZ), including Sigma kinematic analysis code47. We then relevant Gaia astrometric and photometric propagated the XYZ positions forward and information and our estimation of the SpT for backward using proper motion values from each source from its CMD position. We also Gaia eDR3 and a zero value for the radial cross-matched our full sample against literature velocity. We then converted XYZ cartesian estimates of mass, effective temperature, radii, coordinates back to ecliptic longitude and bolometric luminosity, metallicity, and log g latitude in intervals of one year and checked for Gaia sources. Table 1 lists all values with whether the host stars had entered/exited the respective catalog references for the parameter ETZ. We used the visualization tool called noted39,44–46. OpenSpace48 to examine the sample as we To identify stars that have seen and will see proceeded with iterations. Using that software Earth as a transiting planet, we propagate we could move time forward and backward and motions backward and forward in time. To do see how the sample changed in position over this, we used the RA, DEC, parallax, μra, and time. In this way we could visually confirm all μdec values from the GCNS. Note that we did stars. Using this iterative method, we found not eliminate sources that were CMD or 2,034 potential exoplanet host stars in the ETZ RUWE outliers (see Table 1 for detailed in the past, present, and future. information). We used the full 100pc sample Given that all stars are in motion around the and iterated in 1-year bins backward and center of the Galaxy, over time, the linear forward in time. We list all astrometric projection of a star's motion will deviate information, including RUWE, in Table 1 so significantly from the circular orbit a star is the reader can decide on a criterion of their taking around the Galaxy. The solar system's choosing for follow-up of the sample. estimated time to complete one orbit is Ideally, to conduct this analysis, we would ~250Myr. Using a geometric approximation use full spatial and velocity information we find that after ~150Myr, the deviation (XYZUVW). However, only a small between the linear approximation and the subsample of objects in Gaia eDR3 have radial circular orbit would approach the size of the velocity measurements. Therefore, we had to ETZ window. Therefore, we chose a proceed with only tangential velocities and the conservative +/-5000-years period for our projected positions across the sky over time for analysis. This timeframe both conservatively this work. While we cannot account for sources accommodates the projected positions over moving toward or away from us, in a short time time and includes a critical portion of human frame analysis, the tangential motion of a development on Earth. Furthermore, +/- source across the sky drives the impact on its 5,000years ensures that we cover ample time

2 for another nominal civilization to have "genesis" after a star's death50. detected and studied Earth through the rise of Stars with a vantage point that could see modern human civilization. Earth transit - and thus see an interesting planet We approximated the ETZ and rETZ for deliberate broadcasts - are priority targets entrance and exit windows' sizes using the for searches for life and extraterrestrial uncertainties in astrometric parameters. For intelligence1–4. First observations of such stars each of the 2034 stars in our sample, we ran 100 have recently been started11,12. Also, NASA's iterations of their position propagated using TESS has entered the extended mission phase, randomly sampled one sigma uncertainties in with a plan to observe stars across the ecliptic, RA, DEC, parallax, μra, and μdec. We evaluated including those with a vantage point to see the ETZ and rETZ entrance and exit points for Earth transit. each of the 100 iterations and reported the standard deviation of the sample as the window References Methods uncertainties in Table 1. The majority of stars 38. Ricker, G. R. et al. Transiting Exoplanet Survey have such small astrometric errors that the Satellite (TESS). in Space Telescopes and windows have negligible uncertainties. Instrumentation 2014: Optical, Infrared, and Moreover, with 43% of objects spending more Millimeter Wave (eds. Oschmann Jacobus M., than 10,000years in the ETZ, many stars enter J., Clampin, M., Fazio, G. G. & MacEwen, H. A.) 9143, 914320 (2014). and exit the ETZ well before/after our analysis. 39. Stassun, K. G. et al. The Revised TESS Input The length of time that any given star Catalog and Candidate Target List. Astron. J. within 100pc stays in the ETZ in our analysis 158, 138 (2019). is proper motion, which generally scales with 40. Faherty, J. K. et al. A Late-type L Dwarf at 11 distance. Thus, sources closer to the Sun have pc Hiding in the Galactic Plane Characterized generally larger proper motion values than Using Gaia DR2. Astrophys. J. 868, 44 (2018). those further away. For instance, the median 41. Caselden, D. et al. WiseView: Visualizing total proper motion for the 30pc sample in the motion and variability of faint WISE sources. GCNS is about 300 mas yr-1, whereas the ascl ascl:1806.004 (2018). median value for objects out to 100pc is about 42. Gagné, J. & Faherty, J. K. BANYAN. XIII. A 85 mas yr-1. That means that sources closer to First Look at Nearby Young Associations with Gaia Data Release 2. Astrophys. J. 862, 138 the Sun will move through the ETZ faster than (2018). those at larger distances. We find that over 43. Smart, R. L. et al. The Gaia ultracool dwarf these 10,000years, only 78 objects complete an sample – II. Structure at the end of the main entrance and exit. Most stars in our sample sequence. Mon. Not. R. Astron. Soc. 485, (1,954) have either entered the ETZ before our 4423–4440 (2019). analysis started or entered the ETZ and will 44. Muirhead, P. S. et al. A Catalog of Cool Dwarf stay beyond our analysis timeframe. Targets for the Transiting Exoplanet Survey 109 of the objects in our catalog are White Satellite. Astron. J. 155, 180 (2018). dwarfs, dead stellar remnants. While most 45. Anders, F. et al. Photo-astrometric distances, searches for life on other planets concentrate on extinctions, and astrophysical parameters for main sequence stars23,34,35, the recent discovery Gaia DR2 stars brighter than G = 18. Astron. Astrophys. 628, A94 (2019). of a giant planet around a white dwarf19 opened 46. Cifuentes, C. et al. CARMENES input the intriguing possibility that we might also catalogue of M dwarfs. Astron. Astrophys. 642, 16–20,49 find rocky planets orbiting evolved stars . A115 (2020). Characterizing rocky planets in the Habitable 47. Malo, L. et al. Bayesian Analysis to Identify Zone of a WD would answer intriguing New Star Candidates in Nearby Young Stellar questions on lifespans of biota or a second Kinematic Groups. Astrophys. J. 762, 88 (2013).

3 48. Bock, A. et al. OpenSpace: A System for Data and code availability Astrographics. IEEE Trans. Vis. Comput. All data is available in the supplement material. Graph. 26, 1–1 (2019). In addition, all data and code can be found on 49. Kozakis, T. & Kaltenegger, L. High-resolution https://github.com/jfaherty17/ETZ. Spectra of Earth-like Planets Orbiting Red Giant Host Stars. 160, 225 (2020). Competing Interests 50. O’Malley-James, J. T., Cockell, C. S., Greaves, J. S. & Raven, J. A. Swansong biospheres II: The authors declare that they have no The final signs of life on terrestrial planets near competing financial interests. the end of their habitable lifetimes. Int. J. Astrobiol. 13, 229–243 (2014). Additional Information Supplementary Information is available for this Acknowledgments paper. L.K. acknowledges support from the Carl Sagan Institute at Cornell and the Breakthrough Stars that can see Earth transit in the +/- Initiative. J.K.F. acknowledges support from 5000-year period. Table 1 lists all the Heising Simons Foundation and the characteristics for stars that can see Earth Research Corporation for Science transit in the +/-5000-year period, sorted by Advancement (Award 2019-1488). This work distance from the Sun. We also cross-matched has made use of data from the European Space our full sample against literature estimates of Agency (ESA) mission Gaia processed by the mass, effective temperature, radii, bolometric Gaia Data Processing and Analysis Consortium luminosity, metallicity, and log g for Gaia DPAC20 sources. Respective catalog references6,39,44-46 https://www.cosmos.esa.int/gaiahttps://www.c for the parameter are noted in Table 1. osmos.esa.int/web/gaia/dpac/consortium Funding for the DPAC has been provided by Exoplanet host stars that can see Earth national institutions, in particular the transit in the +/-5000-year period. Table 2 institutions participating in the Gaia lists the characteristics of the seven known Multilateral Agreement. This research has also stars that can see Earth transit in the +/-5000- used NASA's Astrophysics Data System and year period, that are known exoplanet host the VizieR and SIMBAD databases operated at stars, sorted by distance from the Sun. CDS, Strasbourg, France. Correspondence and requests for materials Author contributions should be addressed to L.K. (email: L.K. envisioned the idea of the paper, J.K.F [email protected]). identified the ETZ stars. L.K. and J.K.F composed the manuscript, undertook the Reprints and permissions information is analysis, and discussed the content of this available at www.nature.com/reprints manuscript.