Trappist planets not in habitable zone
Published: 11 April 2017 (GMT+10)
On 3 March 2017, we reported on the February 2017 announcement by NASA of their discovery of seven Earth-sized planets orbiting the red dwarf star Trappist-1.1 These planets (identified in order of their distance from their star as Trappist-1: b, c, d, e, f, g, and h) vary in diameter compared to Earth from 23% smaller to 13% larger, but there the similarity ends and there is very little else about them that is like Earth. In particular, newspaper claims that they are covered in oceans of water are gross speculation, despite hugely imaginative artist’s illustrations, published by NASA.
NASA originally claimed that “three of these planets are firmly located in the habitable zone”.2 However, research has now shown that this is not so, and that none of the Trappist-1 planets are suitable to support life.
Conditions for life
For a planet to be favourable for life, it’s not enough for that planet just to be in the habitable zone of its star. As David Coppedge writes in Extrasolar planets: a challenge to biblical cosmology?:
“The star must be quiescent, not subject to large flares or excessive stellar winds. The planet’s orbit must be nearly circular, and not given to surges in obliquity (tilt). The planet must be chemically and thermodynamically constructed to support water. The planet’s star must have the right chemistry to avoid giving off excessive ultraviolet light, which would be deadly to life on its planet. About a dozen factors are now seen as constraining the ‘habitable zone’ to a small fraction of stars.
“Another problem is that about 80% of all stars are red dwarfs. These are about a third as large and about a thousandth as bright as our sun, so we can’t see them with the naked eye. For a planet to be in a red dwarf’s habitable zone, it would need to be so close that it would be tidally locked, i.e. one side always facing the star. So one side would be frying in perpetual day and the other freezing in perpetual night.”
Trappist-1 is unlike our Sun, with an effective temperature of only ~2560 K (i.e. above absolute zero = ~2290 ºC), compared with 5777 K for our Sun. It is only marginally larger than Jupiter, its shine is about 2,000 times fainter than that of the Sun, and most of its radiation is in the near infrared. Hence it has been labelled ‘ultracool’, and the planets are most likely tidally locked, with the attendant problems as noted above, and others noted in the previous article.
Climate simulation of the Trappist-1 system
The planetary and star data of the Trappist-1 system have now been subjected to 3D climate simulation modelling by Eric Wolf at the Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder.3 Wolf assumed “ocean-covered worlds, with atmospheres comprised of N2, CO2, and H2O, and with orbital and geophysical properties defined from observation.”3
For the three inner planets, simulations were for planet d and extrapolated to planets b and c because these being closer to their star “would be significantly hotter than planet d given identical atmospheric compositions”. Likewise for the three outer planets, simulations were for planet f as it is closer to the star than planets g and h, and so have a better chance of being habitable. Here are the results.
Three inner planets are too hot
It turns out that the three inner planets, b, c, and d, are too close to Trappist-1 to be in the traditional liquid water habitable zone around it (i.e. they are too hot). Wolf writes: “Thus, if water ever existed on the inner planets, they would have undergone a runaway greenhouse and lost their water to space, leaving them dry today.”3
Three outer planets are too cold
Readers will be aware that CO2 is one of the greenhouse gases said to be responsible for global warming of Earth because it reflects radiated heat back to Earth. Wolf used as much as 30 bar CO2 atmospheres in his processing. He writes:
“… for all simulations of planet f temperatures became cold enough that CO2 would condense onto the surface and thus these atmospheres would collapse. Planets g and h … receive considerable less stellar flux than planet f, and thus they too would be unable to escape a snowball state if warmed by CO2 alone. Thus we conclude that planets f, g, and h lie outside the traditional liquid water habitable zone defined by maximum CO2 greenhouse limit.”3
In short, planets f, g, and h are too far away from their star to be in the habitable zone around it, i.e. they are too cold.
Does planet e have water?
The above elimination of planets b, c, d, f, g, and h from the habitable zone leaves planet e as the only possible candidate. So does it have water?
Since he accepts the long-age evolutionary scenario, Wolf says that ultracool dwarf stars may take a billion years to settle into a stable system. While this is happening, such stars would subject any planets to intense radiation, driving them into runaway greenhouse conditions. This could cause a huge loss of any water—up to ~7 Earth oceans for planet d. He concludes: “Thus planet e would have needed an initial water inventory at least several times greater than the Earth presently for it to retain abundant water today.”3
The alternative, he says, is that the planets formed further away from the star where the radiation intensity was much less, and migrated inwards much later. But this has its own problems: they would probably ‘migrate’ all the way and be swallowed by the star in a ‘death spiral’—see Planets and migrating theories.
So given that naturalists do not know how Earth came to have so much water, we await evidence or even suggestions as to how planet e could ever have had up to seven times more. As far as we know for certain there are no Earth-like planets other than the one we live on.
Eric Wolf’s research shows that none of the Trappist-1 planets are suitable to support life, which means the system does not support evolutionary hopes about finding life elsewhere in the universe.
References and notes
- Grigg, R., Ultracool Trappist-1 and its seven planets, creation.com, March 2017. Return to text.
- NASA telescope Reveals Largest batch of Earth-size, Habitable Zone Planets Around a Single Star, nasa.gov.au, February 2017. Return to text.
- Wolf, E., Assessing the Habitability of the Trappist-1 System Using a 3D Climate Model, available at arxiv.org/pdf/1703.05815.pdf on pre-print site arxiv. Return to text.