Is There An Earth Analogue?

Updated with new 2020 data from a March 2013 article in ‘Spaceflight’ by Philip Corneille FBIS

Since the Greek philosophers, humans have speculated about the existence of other worlds beyond our planetary system. However, the search for other planets beyond our Solar System had a dismal start. Now astronomers have made some striking discoveries among the 4000 confirmed exoplanets…

The first claims of exoplanet detection were made by astronomers using astrometry, the precise measurement of stellar positions, to detect possible alien worlds orbiting the 70 Ophiuchi and 61 Cygni binary star systems. In the mid 1970s neither astrometric observation could be duplicated and the apparent orbital deviations were dismissed as observational errors due to flaws in the telescope optics. However, in January 1992, Alexander Wolszcan and Dale Frail announced the first detection of an exoplanet around the pulsar PSR B1257+12 located 1000 light years from the Sun in the constellation Virgo.

A pulsar is a rotating neutron star, the ultra-dense remnant of a mainstream star that died and exploded as a supernova, emitting radio waves into outer space. The astronomers used the pulsar timing radio astronomy technique in order to detect tiny anomalies in the extremely regular millisecond radio pulses. In fact they found two exoplanets and by 1994 independent observations confirmed a third and fourth planet around the neutron star.

Their discovery wasn’t worldwide headline news as these dead worlds orbit in the hostile radiation environment around a cinder star.

Detecting exoplanets around a Sun-like star remained an observational challenge due to the huge planet-star contrast, stellar perturbations, diffraction and instrumental noise. However, the determination of a French-Swiss team of astronomers resulted in the commissioning of a sensitive spectrometer on the 1.93 m reflecting telescope at the Observatoire de Haute- Provence in southern France. In practice, the team used the indirect Doppler spectroscopy technique to measure the radial velocity of the parent star caused by the gravitational pull exerted by any exoplanets orbiting it. After the corrections for Earth’s rotation and revolution around the Sun, a clear pattern emerged in their observations of the wobbling Sun-like star 51 Pegasi.

On 6 October 1995, Swiss astronomers Michel Mayor and Didier Queloz announced the first definitive discovery of an exoplanet orbiting 51 Pegasi at 50 light years from Earth in the constellation Pegasus. Their discovery was confirmed by other teams, equally amazed that 51 Pegasi b pointed out to be a ‘hot Jupiter’ closely circumscribing its parent star every 4.2 days. Subsequent discoveries of Jupiter-mass exoplanets hugging their host star lead to new theories of planet formation and planet migration. In 2003, the dedicated Franco-Swiss team proved that the Doppler technique could be refined by achieving first light with the High Accuracy Radial velocity Planet Searcher (HARPS) echelle spectrograph on the 3.6 m telescope operated by the European Southern Observatory (ESO) at La Silla observatory in Chile.

But since October 2012, this instrument has had the precision to detect Earth-mass exoplanets, such as the discovery of an alien world orbiting the Alpha Centauri B component of the triple-star system that is closest to Earth (4.3 light years) revealed. The major disadvantage of the radial velocity technique is that only the minimum mass of a companion planet can be measured because the orbital inclination of the exoplanet’s orbit remains undetermined. The strength of this planet finding technique lays in the combination with other detection methods.

Ambitious hunting

In September 1999, a team of astronomers lead by David Charbonneau performed followup observations of HD209458b, an exoplanet found by the Doppler survey of a Solar analogue star (HD = Henry Draper catalog) in the constellation Pegasus at 150 light years from Earth, and discovered the planet was transiting its host star. Astronomers started to search for exoplanetary systems which could be viewed nearly edge-on in order to obtain additional information to the radial-velocity data. This modus operandi of studying the transit light curve (reduction in brightness of the star) provides a huge amount of data such as the size of the exoplanet relative to its star, the orientation of the exoplanet’s orbit and the measurement of the transiting exoplanet’s atmosphere.

Although transit observations can be done by modest telescopes, the usage of space-based telescopes yielded the most results.

Beginning in 2007, the European CoRoT (Convection Rotation and planetary Transits) space telescope was continuously monitoring the brightness of nearby stars in two opposite regions in the sky, the constellation Snake during summer and the constellation Unicorn during winter, for more than 150 days each. In February 2009, CoRoT discovered a five Earth masses exoplanet, Corot 7b at 490 light years from Earth, which became the first exoplanet to be unambiguously confirmed as a solid, rocky world by both transit and radial velocity observations.

In March 2009, NASA launched the Kepler space observatory carrying a 0.95 m Schmidt reflector to continuously observe a star field located between the constellations Cygnus (Swan) and Lyra (musical instrument lyre), just above the Galactic plane. Kepler looked for Earth-sized exoplanets orbiting in the habitable zones of their parent stars, defined as the range of orbital distances within which water could pool on the surface of these rocky worlds. The distance to most of the 156,000 stars for which Kepler could detect Earth-analogue planets is from 600 to 3000 light years. Three years later, the Kepler team had discovered 2321 exoplanet candidates highlighting NASA’s dramatic entrance on the scene of exoplanetary research!

However, extensive ground-based photometric follow-up observations are required as an observed dimming is not a guarantee of a transiting planet.

Grazing eclipses of a binary star, variations in stellar radiation output and star spots (similar to sunspots) can mimic a transit. Hence the pace of exoplanet discoveries is governed by the detailed analysis of the light curves and time consuming ground-based followup observations to secure the real nature of detected transiting objects.

NASA’s Kepler mission confirmed 2348 exoplanets (March 2020). The space telescope also found a large number of multi-planet systems among which the compact six-planets Kepler-11 star system in the constellation Cygnus, some 2000 light years from Earth. Although these six worlds orbit outside the habitable zone of their star, the discovery of this remarkable system offered the first opportunity to study the chemistry and dynamics of planetary system formation.

Detecting super-Earths

After decades of extensive exoplanetary research, ‘hot Jupiter’ observations and interpretations have matured and current goals are the detections of Earth-analogue worlds around Sun-like stars. The Medio 2012 programme, clever observational techniques with achievable technological detection limits allowed astronomers to discover super-Earths, defined as exoplanets with masses between two and 10 Earth masses. The process of finding super-Earths is tightly linked to an in-depth understanding of the characteristics of their host star.

On the one hand, astro-seismologists need to precisely determine the physical phenomena in stellar photospheres as the scatter of these stellar noises corresponds to the current limiting factors of astronomical detectors. Understanding these stellar phenomena will benefit future data analysis of higher precision instruments output.

On the other hand, stellar mass matters as smaller stars have smaller planets and dim more during a transit. About 75% of the stars in the Solar neighbourhood are low mass cool M-dwarfs. Extrapolation of confirmed data indicates that 40% of these red dwarf stars have a super-Earth class exoplanet orbiting in their habitable zone, although this zone lies too close to the violent flare star for life to exist. So, planet-hunting teams turned to orange K-dwarfs, like Alpha Centauri B in the closest neighbouring stellar system to ours. In October 2012, the HARPS team announced the discovery of an Earth-mass exoplanet orbiting Alpha Centauri B at a distance of 4.3 light years from the Sun. Named Alpha Centauri Bb, it has 1.13 Earth masses and orbits very close to the host star, making it too hot to be habitable.

Nearly three decades of discoveries by ground-based observatories and space telescopes provided the statistical basis to suggest that exo-Earths should be common in our Galaxy. Detectors at telescopes used in follow-up observations are being pushed to a new level of sensitivity to search for Earth-mass exoplanets. Although current space telescopes search for potentially habitable planets, they do not search for life.

Moreover Earth-mass exoplanets are not by default (habitable) Earth-analogue exoplanets. The future of hunting and characterising small rocky Earth-analogue exoplanets will strongly depend on the next generation of space telescopes with instrumentation to directly observe and analyse the atmospheres and biosignatures of these worlds.

Characterising Earth clones

In March 2020, the existence of 4000 exoplanets has been established and advances in observations allow us to explore their physical properties, especially atmospheric composition in greater detail. Astronomers can measure a transiting exoplanet’s atmosphere by spectroscopic filtering of the combined light of the planet-star system during ingress, during transit, during secondary eclipse (anti-transit) and during egress.

A large number of of gas giant exoplanet atmospheres have been studied by the infrared and spectroscopic instruments on board the Spitzer Space Telescope and Hubble Space Telescope. Although their are no official definitions for exoplanets, astrophysicist David Sudarsky set up a theoretical model-based classification for exo gas giant atmospheres.

However, studying the thin atmospheres of transiting small exo-Earths will be more challenging compared to the spectroscopic analysis of the thick atmospheres of ‘hot Jupiter’ exoplanets. Moreover, given the likely diversity of Earth-analogues, the most difficult part will be the accurate interpretation of non-biological processes in these exotic exoplanetary atmospheres. Detecting bio-signatures, gases that can be produced by life forms, remains a challenge as these can also be produced by geophysical and photochemical processes.

Scientists have been developing terrestrial exoplanet atmosphere models based on the data obtained by interplanetary spacecraft which performed remote sensing observations of Earth during gravity assist manoeuvres en route to their celestial targets.

These included Voyager 1 (Sept 1977 and Dec 1990), Galileo (Dec 1990 and Dec 1992), Clementine (March 1994), Mars Global Surveyor (May 2003), Mars Express (July 2003), Mars Reconnaissance Orbiter (Sept 2005 and Oct 2007), MESSENGER (Aug 2005 and May 2010), Venus Express (Nov 2005), Rosetta (Nov 2007 and Nov 2009) and the Deep Impact flyby probe, renamed EPOXIEPOCh (Extrasolar Planet Observation and Characterization – May 2008).

Direct imaging techniques to detect nearby exoplanets using spectroscopic analysis involves the next generation of 30m ground-based observatories. These new telescopes constitute a quantum leap advance in sensitivity for light curve analysis and atmospheric measurement.

For the foreseeable future, exoplanetary research will remain the most exciting and fastest-growing field in astrophysics, allowing the scientific search for habitable worlds and possible life beyond our Solar System. The discovery of life beyond our Solar System would be the most exciting result in the history of science, and would have enormous philosophical implications. Almost 500 years since the time of Nicolaus Copernicus, humanity is on the verge of the greatest change in perspective of our place in the universe!

Lessons learned about Earth’s habitability will be applied to the study of Earth-analogue exoplanets. In the near future, NASA’s 6.5 m James Webb Space Telescope (JWST) should allow spectroscopic follow-up observations of Earth-like exoplanets around nearby stars in order to characterise their atmospheric composition. JWST’s instruments are optimised for observations at infrared wavelengths, which make the spectral lines for carbon dioxide, methane and water vapour stand out. Moreover, JWST will herald the systematic search for exomoons by looking for distortions in the transit signal. Transit timing and duration variations might uncover one or more Earth-sized exoplanets captured as an exomoon around a gas giant exoplanet.

Within the next decade, astronomers will learn how common Earth-analogue exoplanets within the habitable zone of their host stars really are. This knowledge will allow the research to extend the current ground-based wide-field surveys into space in order to find transiting exo-Earths orbiting relatively nearby bright stars.

The Transiting Exoplanet Survey Satellite (TESS) was launched in 2018, the PLAnetary Transits and Oscillations of stars (PLATO) will launch in 2026 and still planned by NASA is the Wide Field Infra Red Survey Telescope WFIRST). WFIRST will use both microlensing and direct imaging techniques to detect nearby exoplanets to facilitate spectroscopic analysis with the next generation of 30m ground-based observatories. These new telescopes will constitute a quantum leap advance in sensitivity for light curve analysis and atmospheric measurement.

For the foreseeable future, exoplanetary research will remain the most exciting and fastest-growing field in astrophysics, allowing the scientific search for habitable worlds and possible life beyond our Solar System. The discovery of life beyond our Solar System would be the most exciting result in the history of science, and would have enormous philosophical implications.

In the 500 years since the time of Nicolaus Copernicus, we are on the verge of the greatest change in perspective of our place in the universe!

Postscript: In November 2013, astronomers reported, based on Kepler space mission data, that there could be as many as 40 billion Earth-sized planets orbiting in the habitable zones of Sun-like stars and red dwarfs in the Milky Way, 11 billion of which may be orbiting Sun-like stars.

A 2015 review concluded that the exoplanets Kepler-62f, Kepler-186f and Kepler-442b were likely the best candidates for being potentially habitable. These are at a distance of 1200, 490 and 1120 light-years away, respectively. Of these, Kepler-186f is similar in size to Earth with a 1.2-Earth-radius measure and it is located towards the outer edge of the habitable zone around its red dwarf. The potentially habitable planet TOI 700 d is only 100 light years away.

This painting by Lynette Cook shows the multi exoplanet system around Cancri 55, a Solar-like star in the constellation Cancer at 41 light years from Earth. The hunt for life-sustaining Earth analogue exoplanets is now underway in earnest.

Lynette Cook/

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