B.J. Fulton/Univ. of Hawaii, Caltech, Ames/NASA
Small worlds come in two flavors — super-Earths and mini-Neptunes.
This conclusion emerges from data collected by the Kepler space telescope. It was charged with hunting for alien planets, meaning those outside our solar system. Now Kepler’s initial mission is over and its data in-hand.
Scientists released Kepler's final tally of so-called exoplanets June 19 at a news conference. The spacecraft has turned up 4,034 of these candidate planets. Among them are 49 rocky worlds, including 10 newly discovered ones. These sit in their stars' Goldilocks zones. That means they fall within a region that’s not too hot and not too cold to support life as we know it. To date, 2,335 of the candidates have been confirmed as planets. That includes about 30 rocky worlds that are in potentially habitable zones.
Benjamin Fulton studies these alien worlds. He works at the University of Hawaii at Manoa and at the California Institute of Technology (Caltech) in Pasadena. He and his colleagues made careful measurements of the candidate planets’ stars. This turned up something unexpected. Few planets had a radius more than 1.5 times that of Earth but less than twice as big as Earth’s.
This split the planet into two types, based on size. Rocky ones, like Earth, had the smaller radii (under 1.5 times the size of Earth's). Gassy planets (the Neptune-like ones) tended to have a radius that was from 2 to 3.5 times the size of Earth's.
“This is a major new division in the family tree of exoplanets,” Fulton reports. It is “somewhat analogous to the discovery that mammals and lizards are separate branches on the tree of life,” he says.
The Kepler space telescope launched in 2009. It had one ultimate goal: to identify the fraction of stars like the sun that host planets like Earth. To do this, it stared at a single patch of sky in the constellation Cygnus for four years. Kepler watched sunlike stars for telltale dips in brightness. Such dips point to when a planet passes in front of its star. Known as a transit, one might think of it as a mini or partial-eclipse.
The Kepler team has still not yet calculated what share of the sun-like stars in Kepler's eye host planets in the Goldilocks zone. But astronomers are confident that they finally have enough data to do so, said Susan Thompson. She is an astronomer at the SETI Institute in Mountain View, Calif. She presented the new data during the Kepler/K2 Science Conference IV being held at NASA’s Ames Research Center in Moffett Field, Calif. (K2 refers to Kepler’s second mission. It began when the telescope’s stabilizing reaction wheels broke.)
Thompson and her colleagues ran the Kepler dataset through “Robovetter” software. It acted like a sieve to catch all of the potential planets that the dataset contained. Running fake planet data through the software pinpointed how likely it was to confuse other signals for a planet or to miss true planets.
“This is the first time we have a population [of exoplanets] that’s really well-characterized,” Thompson says.
Astronomers’ knowledge of exoplanets is only as good as their knowledge of the planets' host stars. So, in a separate study, Fulton and his colleagues turned to the Keck telescope in Hawaii. They used it to precisely measure the sizes of 1,300 planet-hosting stars that were in the Kepler telescope’s field of view. That let them compare the dips in light due to a planet crossing in front of its star to that star’s real size. Those star sizes helped pin down the sizes of the planets with four times more precision than ever before.
The split in planet types the team found could come from small differences in the planets’ sizes, compositions and distances from their stars. Young stars emit powerful winds of charged particles. These winds can blow a growing planet’s atmosphere away.
Bigger planets have more gravity, which helps them hold on to a thicker atmosphere. If a planet was too close to its star or too small to hold its atmosphere tightly — less than 75 percent larger than Earth — it would lose its atmosphere and end up in the smaller group. The planets that look more like Neptune today either had more gas to begin with. Or, they grew up in a gentler environment, Fulton now concludes.
That split could have implications for the abundance of life in the Milky Way. That’s our galaxy. Consider the surfaces of mini-Neptunes, if they exist. They would suffer under the crushing pressure of a thick atmosphere.
“These would not be nice places to live,” Fulton said. “Our result sharpens up the dividing line between potentially habitable planets and those that are inhospitable.”
Better telescopes will sharpen the dividing line even further. Two such telescopes are slated to launch in 2018. The Transiting Exoplanet Survey Satellite will fill in the details of the exoplanet landscape with more observations of planets around bright stars. The James Webb Space Telescope will be able to check the atmospheres of those planets for signs of life.
“We can now really ask the question, ‘Is our planetary system unique in the galaxy?’” says Courtney Dressing. She is an exoplanet astronomer at Caltech. “My guess is the answer’s no. We’re not that special.”
alien A non-native organism. (in astronomy) Life on or from a distant world.
atmosphere The envelope of gases surrounding Earth or another planet.
colleague Someone who works with another; a co-worker or team member.
exoplanet Short for extrasolar planet, it’s one that orbits a star outside our solar system.
galaxy A massive group of stars bound together by gravity. Galaxies, which each typically include between 10 million and 100 trillion stars, also include clouds of gas, dust and the remnants of exploded stars.
Goldilocks zone A term that astronomers use for a region out from a star where conditions there might allow a planet to support life as we know it. This distance would be not too close to its sun (otherwise the extreme heat would evaporate liquids). It also can’t be too far (or the extreme cold would freeze any water). But if it’s just right — in that so-called Goldilocks zone — water could pool as a liquid and support life.
habitable A place suitable for humans or other living things to comfortably dwell.
Jupiter (in astronomy) The solar system’s largest planet, it has the shortest day length (10 hours). A gas giant, its low density indicates that this planet is composed of light elements, such as hydrogen and helium.
Kepler Space Telescope A NASA mission to search for exoplanets — planets beyond the solar system — especially ones that might be Earth-like. The mission’s development began in 2002, by placing the first orders for the needed instruments that would be used. The mission was named for Johannes Kepler (1571 to 1630), the first person to describe the motions of planets about the sun so that their positions could be predicted accurately. The spacecraft carrying the telescope Kepler spacecraft lifted off March 6, 2009, at 10:49 p.m. from the Cape Canaveral Air Force Station in Florida. As of June 2017, it had turned up 2,335 confirmed exoplanets and almost 1,700 more possible ones.
light-year The distance light travels in one year, about 9.48 trillion kilometers (almost 6 trillion miles). To get some idea of this length, imagine a rope long enough to wrap around the Earth. It would be a little over 40,000 kilometers (24,900 miles) long. Lay it out straight. Now lay another 236 million more that are the same length, end-to-end, right after the first. The total distance they now span would equal one light-year.
Milky Way The galaxy in which Earth’s solar system resides.
Neptune The furthest planet from the sun in our solar system. It is the fourth largest planet in the solar system.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and it must have cleared other objects out of the way in its orbital neighborhood. To accomplish the third feat, it must be big enough to pull neighboring objects into the planet itself or to sling-shot them around the planet and off into outer space.
pressure Force applied uniformly over a surface, measured as force per unit of area.
radius (pl. radii) A straight line from the center to the circumference of a circle or sphere.
SETI An abbreviation for search for extraterrestrial intelligence, meaning life on other worlds.
solar system The eight major planets and their moons in orbit around our sun, together with smaller bodies in the form of dwarf planets, asteroids, meteoroids and comets. Or a similar system of celestial objects orbiting one or more related stars.
star The basic building block from which galaxies are made. Stars develop when gravity compacts clouds of gas. When they become dense enough to sustain nuclear-fusion reactions, stars will emit light and sometimes other forms of electromagnetic radiation. The sun is our closest star.
statistics The practice or science of collecting and analyzing numerical data in large quantities and interpreting their meaning. Much of this work involves reducing errors that might be attributable to random variation. A professional who works in this field is called a statistician.
sun The star at the center of Earth’s solar system. It’s an average size star about 26,000 light-years from the center of the Milky Way galaxy. Or a sunlike star.
telescope Usually a light-collecting instrument that makes distant objects appear nearer through the use of lenses or a combination of curved mirrors and lenses. Some, however, collect radio emissions (energy from a different portion of the electromagnetic spectrum) through a network of antennas.
transit (in astronomy) The passing of a planet across the face of a star, or of a moon or its shadow across the face of a planet.
Journal: B.J. Fulton. The California-Kepler Survey. III. A gap in the radius distribution of small planets. Kepler/K2 Science Conference IV, Moffett Field, Calif., June 22, 2017.
Journal: S. Thompson. Kepler’s final exoplanet catalog. Kepler/K2 Science Conference IV, Moffett Field, Calif., June 19, 2017.
Journal: B.J. Fulton et al. The California-Kepler Survey. III. A gap in the radius distribution of small planets. arXiv:1703.10375v2. Posted June 16, 2017.