Bakersfield Night Sky — March 4, 2017
by Nick Strobel
Some columns take a bit of time to figure out what astronomy discoveries I want to highlight but the discovery of seven Earth-sized exoplanets around the nearby star, TRAPPIST-1, made this column easy because the discovery has captured a lot of people’s attention. The day after the announcement, I was interviewed on the Ralph Bailey radio show about the exoplanets and the next day at a statewide workshop, one of the presenters asked me about them. He also asked about whether I thought Pluto was a planet. More about that later.
TRAPPIST-1 is a very cool star slightly larger in diameter than Jupiter but over eighty times the mass of Jupiter, so it is able to fuse hydrogen together to make helium plus energy in its core. The star is almost 40 light years away, so it is relatively nearby as far as stars go but it would take over 700,000 years in our fastest spacecraft to get there. Being so small and cool, it would take a telescope between the size of the two largest telescopes on Mt. Wilson, or eighty inches in diameter, to see it with your eye at the eyepiece. Smaller telescopes with advanced CCD cameras and longer exposures than one-tenth second can detect it, such as the Transiting Planets and Planetesimals Small Telescope–South (TRAPPIST) that found three exoplanets around 2MASS J23062928-0502285, now known by the easier to pronounce TRAPPIST-1.
The 23.5-inch diameter TRAPPIST-South in Chile found three Earth-sized exoplanets in mid-2016 by noticing the star’s brightness drop a tiny amount when the exoplanets passed in front of it. Follow-up work by the Spitzer Infrared Space Telescope found that the three exoplanets were actually seven exoplanets. This is the first system we know of that has seven Earth-size planets orbiting a single star. Our solar system has just two!
Furthermore, three of the exoplanets (e, f, g) are definitely within the star’s habitable zone, the region around a star where the surface temperature on a planet would be not too hot nor too cold for liquid water to exist on its surface. Depending on the composition and thickness of the atmospheres enveloping the other four exoplanets, they might also have nice enough conditions for liquid water to exist on their surfaces too.
Since TRAPPIST-1 is so much cooler than the sun, the exoplanets have to huddle very close to the star to stay warm. Their orbits are between 1.02 million 5.58 million miles from the star. Mercury orbits about 36 million miles from the sun. Because the TRAPPIST-1 exoplanets are so close to each other, astronomers were able to measure the slight changes in their motions as they passed near to each other and thereby determine their masses. With mass and size information, the densities of the worlds can be determined and that tells us that these are rock-iron worlds. Corroborating that are observations with the Hubble Space Telescope of the two innermost exoplanets that showed that they do not have puffy atmospheres like a Neptune or Uranus.
Being so close to their star, the exoplanets are very likely tidally locked to the star so that they keep one side facing the star. Years ago we thought being tidally locked would make it impossible for an atmosphere to exist because the night side would be so cold that the atmosphere would freeze out. More advanced computer models of planet atmospheres showed that an atmosphere could still exist with perpetual strong winds blowing between the two sides of the exoplanets. A temperate location would be near the day-night boundary, so any lifeforms that exist would see their star near the horizon all the time. Another thing the inhabitants would see is the neighboring planet sometimes appearing larger in the sky than our moon in our sky.
There will, of course, be plenty of further observations of this system by Hubble, Spitzer, and the Kepler planet-hunting space telescope. Kepler’s observations of the system will conclude within a few days from now. The James Webb Space Telescope, scheduled to launch in 2018, will be able to detect the spectral lines (chemical fingerprints) of water, methane, oxygen, ozone, and other molecules in the exoplanet atmospheres. Webb will also be able to determine the exoplanet temperatures and surface pressures.
Closer to home, scientists on the New Horizons mission are making a push to have Pluto regain its “planet” status. They clearly feel stung by the reclassification of Pluto as a “dwarf planet” and use words like “demotion” to describe what happened to Pluto in 2006. One problem with their proposed new definition—a planet is anything smaller than a star but large enough to make itself round—is that there would then be at least 110 planets in our solar system. Good luck to the school kids having to memorize all of their names!
In this evening’s sky, we’ll see the first quarter moon cover up the orange-red eye of Taurus (Aldebaran) between 7:08 and 8:25 p.m. (see the first chart below). Other parts of the U.S. will have varying times of the moon covering Aldebaran. The Thursday after daylight savings begins, March 16, I will give an encore showing of my “Mars Travel Guide”. Tickets are on sale now.
Want to see more of the stars at night and save energy? Shield your lights so that the light only goes down toward the ground. See www.darksky.org for how.
Director of the William M Thomas Planetarium at Bakersfield College
Author of the award-winning website www.astronomynotes.com