Astronomers earlier this month (April 6, 2012) announced evidence for a record-breaking nine planets around the nearby sun-like star HD 10180. Results from 2010 had shown that the star has at least five planets, and possibly as many as seven. The new results – proposing a nine-planet model for this solar system – would make HD 10180 the first star to surpass the number of planets in our own solar system and suggest that our sun is not alone in hosting a large family of diverse worlds.
The planets encompass a wide range of sizes and environments. The smallest planet is only 30% more massive than Earth making it most likely a rocky world like our own. It is also the closest planet to its star, orbiting once every 29 hours. Such a close orbit means its surface is blasted by intense stellar radiation thus making it completely inhospitable to any life we would recognize. The most distant planet has about two-thirds the mass of Saturn. Completing one trip around the star every six years, its orbit would fit entirely within our own system’s asteroid belt – the rocky debris field between Mars and Jupiter. One planet orbits well within HD 10180’s “habitable zone”. In other words, it is just the right distance from the star for liquid water to exist on it surface. Unfortunately, its Neptune-like mass means the planet is probably a gas giant and therefore lacks the solid surface needed for rivers, lakes, and oceans to form.
Two years ago, the star gained noteriety when astronomers announced the discovery of six planets all roughly the mass of Neptune with hints of a seventh. These new results, described in a paper accepted to the journal Astronomy and Astrophysics on April 6, 2012, have subjected that data to a more rigorous statistical analysis which not only confirms and refines the properties of the six known planets, but also reveals the subtle signatures of three more planets all just a few times the mass of Earth.
The planets were discovered by looking for wobbles in the star’s motion caused by gentle tugging from the orbiting worlds. The star’s wobble is too small to allow astronomers to directly observe the star’s motion on the sky. Researchers instead rely on the Doppler effect: the compressing and stretching of light waves caused by the star moving towards and away from our telescopes. This is the same principle that causes a train horn to apparently change pitch as it races past you. By observing a star whose light waves get repeatedly stretched and compressed, astronomers can infer not only the motion of the star, but also the mass of the object pulling on it. This technique has led to the discovery of over 90% of the 763 known planets around other stars.
The planets were discovered using the High Accuracy Radial Velocity Planet Searcher (HARPS) on the European Southern Observatory’s (ESO) 3.6-meter La Silla telescope. Sitting 2400 meters (7800 feet) above sea level in the southern part of Chile’s Atacama Desert, HARPS works much like a prism, allowing astronomers to split star light into its component wavelengths. The incredible precision of this remarkable instrument has uncovered a wealth of diverse planets since it started operations in 2003.
HD 10180 is star pretty similar to our sun. It’s slightly larger and brighter, though human visitors would probably find comfort in its familiar yellow glow. Astronomers estimate that HD 10180 is pretty old: born approximately 7 billion years ago, it has been around for about half the age of the universe. Too faint to be seen without the aid of a telescope, the star sits about 127 light-years away in the southern constellation of Hydrus, the water snake. This means the light that we currently see started its journey to Earth roughly the same time the Statue of Liberty arrived in New York City and Mark Twain published “The Adventures of Huckleberry Finn”.
Bottom line: Astronomers now believe that nine planets move in orbit around the sun-like star HD 10180. This star now holds the record for the largest number of planets, surpassing our own sun. This discovery supports the hypothesis that large, diverse planetary systems are common in the universe.
Chris Crockett got his Ph.D. in astronomy from UCLA in 2011 and worked at Lowell Observatory and the U.S. Naval Observatory. He then realized he enjoyed talking about astronomy a lot more than actually doing it. After being awarded a Mass Media Fellowship in 2013 by the American Association for the Advancement of Science, he spent a summer writing for Scientific American, then went on to become the staff astronomy writer at Science News from 2014 - 2017. These days, he freelances, focusing on stories about astronomy, planetary science, and physics. His work has appeared in Science News, Scientific American, Smithsonian Magazine, Knowable, Sky & Telescope, and the American Physical Society's online magazine Physics.