A bizarre exoplanet whose orbit takes it from the frozen wastelands of its planetary system to the inner edge of its habitable zone is challenging astronomers’ concepts of what kinds of planets can potentially support life.
The exoplanet HD 20794d orbits 82 Eridani, a magnitude +4.3 star that lies 19.7 light years away in the constellation of Eridanus, the River. It is the third planet to be found in the system – a fourth world thought to have been discovered in 2011 was subsequently ruled out.
The two inner planets, HD 20794b and c, have a little over twice the mass of the Earth and orbit their Sun-like star every 18.3 and 89.6 days, respectively. HD 20794d, which was identified in 2015, has a mass 6.6 times greater than the Earth and orbits its star every 647 days. However, new studies by astronomers using the ESPRESSO and HARPS spectrographs at the European Southern Observatory (ESO) have discovered something highly intriguing about HD 20794d.
The planets in our Solar System all have roughly circular orbits, whereas comets have orbits that tend to be more elongated, bringing them close in to the Sun before they travel far out again. We call the degree of such elongation the ‘eccentricity’ of an orbit. An eccentricity of 0 indicates a perfectly circular orbit, whereas an eccentricity of 1 is a hyperbolic orbit. Mercury, the planet in the Solar System with the least circular orbit, has an eccentricity of 0.2.
HD 20794d is different. We can’t see the planet transit its star, but we can detect the gravitational pull that it exerts on its star, causing the star to wobble around the common centre of mass between star and planet. The closer a planet is to a star, the greater this wobble, which manifests as a ‘radial velocity’ seen in the redshift and blueshift of the star’s light as it wobbles to and fro. Using ESPRESSO (Echelle SPectrograph for Rocky Exoplanets and Stable Spectroscopic Observations) on the Very Large Telescope and HARPS (High Accuracy Radial-velocity Planet Searcher) on ESO’s 3.6-metre telescope, both in Chile, a team led by Xavier Dumusque of the University of Geneva measured the radial velocity and found that HD 20794d’s orbital distance from its star varies quite drastically.
At its furthest point, known as apastron, HD 20794d is two astronomical units (300 million kilometres) from its star. This is farther than the 228 million kilometres that Mars gets from our Sun. Because 82 Eridani has a luminosity just two-thirds of our Sun, its habitable zone is shrunk down compared to our Solar System’s habitable zone, which Mars sits on the outer edge of. HD 20794d goes well beyond the habitable zone, where water on the surface would all be solid ice.
But HD 20794d doesn’t spend all its time out in the cold. Its orbital eccentricity is a huge 0.4. As it moves along its orbit, it gets much closer to its star, entering the habitable zone and moving right to its inner edge at 0.75 astronomical units (69.7 million kilometres), about as close as Venus is to our Sun. As HD 20794d makes this journey, its ice – assuming it possesses any – will melt, forming oceans that might even begin to evaporate and produce a steamy atmosphere when the planet reaches it closest point to the star (a point called periastron). Then, it begins to move away again, and any surface water would re-freeze, ready to begin the cycle again.
Ordinarily a planet in the habitable zone of a Sun-like star less than 20 light years away would be a cause for excitement, but astronomers are not sure what to make of HD 20794d, which only spends some of the time in the habitable zone. Could life survive such temperature extremes? It would depend upon the make-up of the planet, and whether it has an atmosphere thick enough to retain some heat beyond the habitable zone, or perhaps whether habitable niches beneath frozen oceans and tundra could persist during the deep winter. Speaking of seasons, whereas Earth’s seasons are governed by our planets 23.4-degree tilt, HD 20794d’s seasons will be determined solely by where it is in its orbit.
Over 5,830 exoplanets have been confirmed discovered so far, and astronomers are finding that worlds with eccentric orbits are more common than we might have thought based only on our Solar System. These eccentric orbits could be the result of interactions between planets as they were forming and giant planets may have been migrating, marauding their way through space only to be ejected from the system entirely, but not without pushing around other planets. Models suggest that Jupiter and Saturn didn’t move around and bully other planets quite so much, leaving Earth’s orbit and the orbits of the other terrestrial planets intact.
Because habitable zone planets are harder to find, especially around Sun-like stars because they tend to orbit farther away, reducing the radial velocity signal and leaving long gaps between transits, this is therefore the first terrestrial planet known to move in and out of the habitable zone. However, given how commonplace high eccentricities appear to be, it may not be the last of these ‘half-Goldilocks’ worlds to be found. Where that leaves habitability in the wider Universe remains to be seen.
