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Ultracool Trappist-1 and its seven planets

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Published: 3 March 2017 (GMT+10)
NASA Discovery-7-Earth-sized-planets
NASA has released this artist’s concept of Trappist-1 and the seven Earth-sized planets orbiting it.

On 23 February 2017, NASA announced the discovery of “the first known system of seven Earth-sized planets around a single star”.1 The star is Trappist-1 in the constellation Aquarius. According to NASA administrator Prof. Thomas Zurbuchen, “This discovery could be a significant piece in the puzzle of finding habitable environments, places that are conducive to life.”1 So just what has been found, and is it significant for life?

The star Trappist-1

European Southern Observatory comparison-sun-trappist-1
The Sun and Trappist-1 to scale. The ultracool star has only 11% of the diameter of the Sun and is much redder in colour.

Trappist-1 is classified as an ultracool2 red dwarf star. It is 39.5 light years away from Earth (i.e. ~355 trillion km or ~232 trillion miles), and it has about 8% of the mass and 11% of the radius of our Sun. So it is not much larger than Jupiter, although it’s 84 times more massive. NASA says that if our Sun was the size of a basketball, Trappist-1 would be the size of a golf ball. With a claimed temperature for Trappist-1 of 2,550 ± 55 K (i.e. above absolute zero, cf. 5,778 K for our Sun) and its small size, its feeble radiant energy is about 2,000 times fainter than that of our Sun.3 And most of this is in the infrared region of the electromagnetic spectrum, invisible to our eyes.

TRAPPIST is an acronym for a Belgian robotic telescope named TRAnsiting Planets and PlanetesImals Small Telescope,4 located at the European Space Agency’s La Silla Observatory in Chile. In May 2016, researchers using this equipment announced the discovery of three of these planets. On 23 February 2017, astronomers using the NASA Spitzer Space Telescope and the Very Large Telescope at Paranal, Chile, among others, increased the number of claimed planets in this system to seven.1

The planets were identified by the dimming of the light output of Trappist-1 as each of the planets passed in front of it—called the transit or photometric detection method. This required that the plane of these planets’ orbits pointed exactly towards a line of sight on Earth. “The size of each planet was deduced from the amount of starlight it blocked out, while the mass was estimated from the way it was pushed and pulled around by other planets in the system.”3

The seven exoplanets

The seven exoplanets are identified as Trappist-1b, c, d, e, f, g, and h. They are called exoplanets because they orbit a star outside our solar system. They are thought to be rocky, and are labelled ‘Earth-sized’ because they vary in radius from 24% smaller to 13% larger than Earth. However, they are all very close to their star—Mercury, the innermost planet of our Solar System, is six times further away from the Sun than the outermost seventh exoplanet is from Trappist-1. As a result, these seven planets all whip around their star very quickly—in 1.51 days (of 24 hours) for planet b, 6.1 days for planet e, to ~20 days for planet h, cf. about 88 days for Mercury’s orbit around the Sun.

Planets e, f, and g, are said to be in the ‘habitable’ or ‘Goldilocks’ zone. This merely means that these three planets are at a distance from their star where the temperature on the planet surface facing the star is reckoned to be somewhere between 0° and 100° C, so that, if water were present, the temperature would allow it to be liquid. This does not mean that any of the planets contain water, as none has been identified. This, despite NASA’s optimistic release of an artist’s concept of what one of the planets could look like if it contained an ocean of liquid water, and also NASA’s confidently colouring the illustrations of planets e, f, and g, in blue and green hues, thereby suggesting water and vegetation.

Are they habitable?

According to NASA, “The planets may also be tidally locked to their star, which means the same side of the planet is always facing the star, therefore each side is either perpetual day or night.”5 This means that the side of these planets in perpetual day could be too hot for life, while the side in perpetual night could be close to the temperature of outer space (–270°C), unless any of these planets had an independent source of heat such as ongoing volcanic activity (which has not been claimed, and would imply a short age rather than a long age). It would also mean a low or non-existent magnetic field under evolutionary models.

Little else is known about Trappist-1. Red dwarfs in general are known for large stellar flares, bursts of radiation, and large changes in luminosity, because they are often covered by large star spots that can just about halve their radiation output for months. All of these could be detrimental to life on a planet orbiting a red dwarf star,6 most likely including Trappist-1.

Indeed, the article about these planets in Nature states:

“Although at least some fraction of each planet could harbor liquid water, it doesn’t necessarily follow that they are habitable. TRAPPIST-1 emits about the same amount of X-ray and ultraviolet radiation as the Sun does, which could chew away at any protective atmospheres the planets might have.”7

In this case, Trappist-1 may be even less favourable for life than other red dwarfs, which emit hardly any UV.

One other problem about the formation of life on other planets than Earth is that, although liquid water is essential for biochemical reactions within already living cells, water inhibits the interaction of these same chemicals outside the protective membrane of a cell, thereby preventing any alleged evolutionary mechanism for life from taking place. See Origin of Life: the polymerization problem.

Also, water is a very strong absorber of infrared radiation, and the star emits mostly infrared. This would leave very little radiation for any photosynthesis. As far as we know for certain, Earth is the only planet in the entire universe where life exists, and this is because God created life here. It would appear that the main reason for the claim of possible habitability of other planets is the hope on the part of the claimants that Earth is not a special place in the universe—despite scientific evidence that it is.

How do we account for such exoplanets?

Answer: Genesis 1:1 says that “In the beginning, God created the heavens and the earth.” Genesis 1:14–19 tells us that on Day 4 of Creation Week, God created not only our solar system’s sun and moon, but also “the stars”, i.e. the other objects in the universe—both inside and outside of our solar system. Some of these we can see unaided, and others, like Jupiter’s moons or Saturn’s rings, and now Trappist-1 and its planets, with the help of instruments. As man has applied his creative inventiveness to discovering progressively more and more distant objects, there is an ever-increasing array of variety on display to us. As David was already able to say in Psalm 19, verse 1, the heavens really do declare the glory and handiwork of God.

Update: see Trappist planets not in habitable zone.

References and notes

  1. NASA telescope reveals largest batch of earth-size, habitable-zone planets around single star, 23 February 2017. Return to text.
  2. An ultracool dwarf is a stellar or sub-stellar object of spectral class M that has an effective temperature under 2,700 K (~2,430°C; ~4,400°F). Return to text.
  3. Sample, Ian, Science editor, The Guardian, Exoplanet discovery: seven Earth-sized planets found orbiting nearby star, 24 February, 2017. Return to text.
  4. Note: the word ‘planetesimals’ presumes the widely held evolutionary story for how planets supposedly form from a solar nebula. In that scenario, cosmic dust grains are said to collide and stick together until they form bodies larger than 1 km, called planetesimals. These are then presumed to attract each other by mutual gravity to form planets. Return to text.
  5. NASA’s Hubble Telescope Makes First Atmospheric Study of Earth-Sized Exoplanets, 21 July 2016. Return to text.
  6. See Sibley, Andrew, Superflares and the origin of life, J. Creation 31(1):111–115, 2017, section ‘Origin of life on other planets—superflares on brown and red dwarfs’, p. 114. Return to text.
  7. Witze, A., These seven alien worlds could help explain how planets form, Nature News, 22 February 2017. Return to text.

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