New Discovery Raises Big Question of Life’s Cosmic UniquenessArticles
Discovering exoplanets is becoming relatively common. Nearly 2,000 planets that orbit stars other than the Sun have been detected. However, an important new discovery has been announced by a team of astrophysicists based at the Harvard-Smithsonian Center for Astrophysics (CfA), through work funded by the John Templeton Foundation’s Alien Earths Initiative.
The CfA is a major partner with the Geneva Observatory on the HARPS-North Collaboration, which includes several other European partners, and the researchers have found the closest known exoplanet to Earth that transits its star. The discovery could lead to new breakthroughs in advancing one of the most profound big questions: does life exist elsewhere in the universe?
The new exoplanet is in the constellation Cassiopeia, the group of stars familiar to northern hemisphere star-watchers as a W-shaped pattern, often seen high in the night sky. Tucked alongside one of the legs of the W is a modest star named HD 219134. It is visible to the naked eye and somewhat cooler, smaller, and more orange than our Sun. In its planetary system is the newfound exoplanet, named HD 219134b. It passes across the front of the star every 3 days.
Exoplanets that transit their stars are particularly valuable to researchers because they enable observations that promise all sorts of information about these other worlds which, until now, has remained elusive. Added to the transiting is the proximity of HD 219134b: it is just over 21 light years away. For astronomers who are used to finding exoplanets hundreds of light years away, that makes it a near-neighbor. It’s ideal for future study, and high on astronomers’ list of questions will be the nature of its atmosphere. Being able to study the atmospheres of exoplanets is an important step towards searching for the chemical signatures of extraterrestrial life.
The new world was discovered using the HARPS-North instrument on the 3.6-meter Telescopio Nazionale Galileo in the Canary Islands. The method used to detect it allows astronomers to calculate its mass; HD 219134b is a super-Earth, weighing about 4.5 times our celestial home. In April of this year, the team targeted the system with NASA’s Spitzer Space Telescope. As estimated, the star dimmed slightly as the planet crossed the star’s face. Measuring the depth of the transit dimming gave the planet’s size, which is 1.6 times Earth. As a result, the planet’s density can be calculated and this shows it is a rocky world. A paper describing these findings has been accepted for publication in the journal Astronomy & Astrophysics.
“This is a very significant advance for our field. As a community, we crave to study rocky worlds, but the ones that are known orbit distant stars, observationally inaccessible to us. The world we have discovered is much hotter than the Earth and hence isn’t habitable. Nonetheless, by discovering a nearby rocky planet transiting a star, we believe we have opened the door to follow-up studies, notably of its atmosphere, and this is an important milestone on the search for biosignatures,” David Charbonneau of Harvard University explains. Several news outlets, both in the United States and abroad, reported on the finding, including CNN, ABC News, and The Christian Science Monitor.
The discoveries did not stop with HD 219134b. Three additional planets in the system were also detected. The first, weighing at least 2.7 times Earth, orbits the star once every 6.8 days. A second Neptune-like planet with 9 times the mass of Earth circles in a 47-day orbit. And much further out, a hefty world 62 times Earth’s mass orbits with a “year” of 1,190 days. If these planets transit the star, similar observations to those performed for HD 219134b will be possible, promising more data.
The Alien Earths Initiative is a $2,000,000 grant, launched in 2013, that brings together astronomers, planetary scientists, atmospheric scientists, and geobiologists to pursue interdisciplinary research aimed at identifying and characterizing the closest habitable exoplanets. HARPS-North Rocky Planet Search is a dedicated survey examining about 50 nearby stars for signs of small planets.
HD 219134 was one of the closest stars in the sample, so it was particularly lucky to find one that hosts a transiting planet. As it now holds the record for the nearest transiting exoplanet, it is likely to be a favorite for study by researchers for years to come.