• WHY WE EXPLORE
• PLANETARY NEWS
  • Archive
  • Our RSS News Feeds
  • Asteroids and Comets
  • Extrasolar Planets
  • Our Solar System
  • Search for Life
  • Solar Sailing- LightSail
  • Space Missions
  • Space Telescopes
  • The Planetary Society
• SPACE TOPICS
• FOR KIDS

browse projects: Apophis Mission Design Competition Asteroids - The Potential Threat Carl Sagan Fund for the Future Catalog of Exoplanets FINDS Exo-Earths International Year of Astronomy LIFE Experiment: Phobos LightSail - Solar Sailing Mars Climate Sounder Team Website Messages from Earth Microrovers Mirror Bees: Planetary Defense Observing Earth Pioneer Anomaly Planetary Microphones SETI Optical Telescope SETI Radio Searches SETI@home Shoemaker NEO Grants Space Advocacy Space Information Stardust@home

browse space topics: 2001 Mars Odyssey Akatsuki (Venus Climate Orbiter / PLANET-C) Asteroids and Comets Cassini-Huygens Chandra X-Ray Observatory Chandrayaan-1 Chang'e 1 Compare the Planets Curiosity (Mars Science Laboratory) Dawn Deep Impact Earth Extrasolar Planets Genesis Hayabusa (MUSES-C) Hubble Space Telescope IKAROS Jupiter Kaguya (SELENE) Lunar Reconnaissance Orbiter (LRO) Mars Mars Exploration Rovers Mars Express Mars Global Surveyor Mars Reconnaissance Orbiter Mercury MESSENGER Moon Near Earth Objects Neptune New Horizons Past Missions Phoenix Planetary Analogs Planetary Exploration Timelines Pluto and Charon Private Missions Rosetta Saturn Search for Extraterrestrial Intelligence SMART-1 SOHO Space Imaging Spitzer Space Telescope Stardust Trans-Neptunian Objects Ulysses Uranus Venus Venus Express Voyager Weekly Planetary Radio Trivia Contest WISE

Planetary News: Extrasolar Planets (2004)

Only 14 Earth Masses! The Hunt for Other "Earths" Heats Up

Click to enlarge > A "Hot Jupiter" Credit: G. Bacon (STScI/AVL)

It is the shared dream of all planet hunters: finding a small rocky planet, rich in water and teaming with life, far beyond our Solar System. And although the discovery of such a distant “Earth” still remains far beyond the capabilities of current detection methods, significant progress is being made. The announcement this week by a team of European astronomers that they have found a planet only 14 times the mass of the Earth is a sure indication that it is only a matter of time before the detection of new “Earths” will be within scientists’ reach.

The new planet is by far the smallest extrasolar planet ever detected, and it orbits the star Mu Arae every nine and a half days. It joins a Jupiter-sized gas giant that orbits Mu Arae every 650 day period, and another companion object, possibly a planet, orbiting the star much further away. At a distance of only 50 light years, the Mu Arae system is very much in our own galactic neighborhood, and with at least three orbiting objects, including the smallest planet ever detected, it looks more like our Solar System than any planetary system discovered as yet.

The European team is led by Nuno Santos of the University of Lisbon in Portugal, and includes Michel Mayor of the Geneva Observatory, who found the first extrasolar planet in 1995. They made their discovery using the new ultra-sensitive HARPS spectrograph at the 3.6 meter telescope of the European Southern Observatory at La Silla, Chile.

Like the vast majority of the 120 odd planets detected outside the Solar System, the new planet was detected by means of the spectrographic, or “radial velocity” method. In this approach scientists measure the periodic change in a star’s spectrum as it wobbles to the tug of an orbiting planet. The more sensitive the spectrograph, the smaller the wobble that can be detected, the planet that can be detected is smaller as well. The HARPS spectrograph, able to detect a stellar wobble as slight as 1 meter per second, is the most sensitive in the world.

The discovery of the planet came as a surprise to Santos’ team, as they were studying the internal composition of Mu Arae, a star unusually rich in heavy elements. During 8 nights in June of 2004 they closely monitored the spectrographic shifts of the star in an attempt to detect the small acoustic waves that pulsate which cause the surface of a star to periodically pulsate. On analyzing the data they noticed that on top of the expected signature of the acoustic waves there was also an unmistakable nine and a half day cycle in the star’s spectrum. Follow-up observations confirmed the researchers initial suspicions: a there was indeed a planet orbiting close by Mu Arae and completing each revolution in nine and a half days.

Even more importantly, the miniscule amplitude of the star’s nine and a half day wobble suggested that the orbiting planet was small indeed. Detailed calculation revealed that it was about 14 times the mass of the Earth. This made it unmistakably the smallest extrasolar planet ever discovered orbiting a Sun-like star.

Scientists aren’t sure what the new planet is made of. At approximately the size of Uranus, its mass is on the boundary between the rocky planets of the inner Solar System and the Gas giants further out. Nevertheless, based on the planet’s orbit, Santos and his team reason that it is more likely to be a rocky planet than a gas giant. A gas giant would have migrated inwards from its original orbit and, according to existing models, would have grown much larger than the new planet in the process. This suggests that the planet is a “super Earth” that was formed more or less in its present orbit, in the shadow of its star, where no gas giants can form.

Though relatively small, and likely to be rocky, the planet is in other ways an unpromising “Earth.” Orbiting so close to its star it is far too hot to retain any surface water, and highly unlikely to support any form of life. The new planet orbiting Mu Arae is therefore no “Earth,” but it is more like one than any planet previously detected. Its discovery is a clear indication that slowly but surely scientists are closing in on that holy grail of planetary searches.