Earth’s Seven Sisters: Are They Really Similar?

Earth’s Seven Sisters: Are They Really Similar?

NASA just announced the discovery of the largest ever batch of Earth-size, habitable-zone planets around a single star, 39 light-years away. The star is TRAPPIST-1, otherwise known by its colorful name, 2MASS J23062928-0502285. The TRAPPIST (Transiting Planets and Planetesimals Small Telescope) team has discovered seven “Earth-sized” planets orbiting TRAPPIST-1, of which three are labeled as “habitable.”

Just how Earth-sized and habitable are these planets? Details about the features of the seven planets and their host star were published on February 22 in the British science journal Nature.1

Host Star’s Features
The host star for the seven planets is far from Sun-like. In fact, it is about as far from being Sun-like as one can get. TRAPPIST-1 is ultracool. At visual wavelengths, its luminosity = 0.00000373 times that of the Sun! If it were any cooler, it would cease to be a star. Its mass = 0.080±0.009 times that of the Sun (just 83 times greater than Jupiter’s mass). This is the lowest mass a star can possess and still be a star. The Sun’s equatorial rotation rate is 25.4 days. TRAPPIST-1’s rotation rate is just 1.4 days.

Ultracool, fast-rotating stars exhibit frequent huge flares that are accompanied by strong x-ray and ultraviolet radiation. The XMM-Newton x-ray space telescope has confirmed that the planets orbiting TRAPPIST-1 that reside in the liquid water habitable zone are exposed to sufficient x-ray and ultraviolet radiation from TRAPPIST-1 to significantly chemically alter and erode their primary and secondary atmospheres.2

Owing to how extremely cool TRAPPIST-1 is, the zone where liquid water could conceivably exist (the liquid water habitable zone) on the surface of a planet orbiting it is just 2.3–4.6 million miles away. For comparison, Earth orbits the Sun from 92.7 million miles away. All seven of TRAPPIST-1’s planets orbit it from less than 6 million miles away. Being that close to TRAPPIST-1, they are tidally locked to TRAPPIST-1 in the same manner that the Moon is tidally locked to Earth. That is, the gravitational pull of TRAPPIST-1 on the near side of its seven planets compared to the far side is sufficiently stronger to force one side of each of the seven planets to always face TRAPPIST-1.

Planets’ Features
This tidal locking means that the star-facing side of each planet is perpetually illuminated, whereas the opposite hemisphere is perpetually dark. Therefore, one hemisphere of the planet will be blazing hot while the opposing hemisphere will be extremely cold. Only in the twilight zone boundary between perpetual light and perpetual darkness will surface liquid water be possible. This possibility presumes that for each planet the twilight edge will not move. Given how close the planets are to one another, it is inevitable that the twilight edge on each planet will move. Thus, realistically none of TRAPPIST-1’s planets are likely to ever possess any surface liquid water.

The seven planets orbiting TRAPPIST-1 are Earth-sized in the same sense that Venus and Mars are approximately Earth-sized. Compared to Earth’s mass, TRAPPIST-1’s planets possess masses = 0.85, 1.38, 0.41, 0.62, 0.68, 1.34, and unknown, respectively. It would be more accurate to say that TRAPPIST-1’s seven planets are likely to be rocky like Earth. However, to say that they are Earth-sized is a stretch. Venus is closer to Earth’s mass than six of TRAPPIST-1’s planets. The seventh planet, which is closer to Earth’s mass, is also nearest to TRAPPIST-1, just a million miles away, far too close for it to be conceivably habitable.

All seven of TRAPPIST-1’s planets orbit it much closer than Mercury orbits the Sun. The periods of revolution about TRAPPIST-1 (the planet’s years) for the seven planets are 1.51, 2.42, 4.05, 6.10, 9.21, 12.35, and 20 Earth days, respectively. These orbital periods are near resonant. In the same time that the innermost planet makes eight orbits, the next three more distant planets make five, three, and two orbits. This orbital synchrony means that these four planets exert periodic gravitational influences on one another. These periodic gravitational influences rule out the possibility of life on these planets.

The orbital architecture of TRAPPIST-1’s seven planets indicates that the planets formed farther away and migrated inward.3 Such migration and the proximity of the planets to one another means that they are unlikely to possess the atmospheres and the ingredients necessary for life.

In one of the papers published in Nature, the astronomer-author wrote that “finding seven transiting Earth-sized planets in such a small sample suggests that the Solar System with its four (sub-) Earth-sized planets might be nothing out of the ordinary.”4 What apparently is very much out of the ordinary is that our solar system possesses eight planets with highly fine-tuned characteristics that make possible life on one of its planets. You can read more about these highly fine-tuned characteristics in my book Improbable Planetand in this previous blog post.

  1. Ignas A. G. Snellen, “Astronomy: Earth’s Seven Sisters,” Nature 542 (February 2017): 421–23, doi:10.1038/542421a; Michaël Gillon et al., “Seven Temperate Terrestrial Planets around the Nearby Ultracool Dwarf Star TRAPPIST-1,” Nature 542 (February 2017): 456–60, doi:10.1038/nature21360.
  2. Peter J. Wheatley et al., “Strong XUV Irradiation of the Earth-Sized Exoplanets Orbiting the Ultracool Dwarf TRAPPIST-1,” Monthly Notices of the Royal Astronomical Society Letters 465 (October 2016): L74–L78, doi:10.1093/mnrasl/slw192.
  3. Gillon, “Seven Temperate Terrestrial Planets.”
  4. Snellen, “Astronomy,” 421.
  5. Hugh Ross, Improbable Planet: How Earth Became Humanity’s Home (Grand Rapids: Baker, 2016), 28–83.