New King of the Galaxy Crowned

The Bad Astronomer (found at Google+, Twitter, Facebook and of course the Bad Astronomy Blog) leads on the story of the star HD 10180, which is a star you can't see with the naked eye, but is readily seen with a telescope. It is in the Hydrus constellation, if your are familiar with your constellations (Hydrus has four stars with planets!). According to a reassessment of the data concerning this star, it seems it possesses nine planets. If you are counting, that is one more planet than our own solar system (or Sol System, if you prefer) contains, if you accept Pluto's current status (RIP Pluto). So, the score so far on the solar system with the most planets is...

HD 10180         9 Planets          Status: King of the Galaxy
Solar System    8 Planets           Status: Runner-up
Kepler 11          6 Planets           Status: Never wins at anything



HD 10180 is a star very much like our own sun, a G-type star. It is only a little bit larger than our sun, as far as stars are concerned (6% larger by mass, 120% larger by radius, 150% brighter than our sun). However, it is a yellow star. It converts hydrogen to helium, just like our own sun. It even has a similar rotation period of about 24 days, where our sun takes about a month. And just like our sun, HD 10180 has planets. How many planets?

At first glance, 6 planets. These planets were initially discovered using the Radial Velocity Method (aka - the Doppler Spectroscopy Method). This method consists of watching a star and measuring its exact light output. If the light output varies, and especially varies on a regular basis and in identical amounts, then a planet is likely orbiting its parent star and causing that star to wobble. Those wobbles cause a Doppler effect in that star's light output in much the same way that the sound of a car changes if its coming toward you or moving away from you. In this way, HD 10180 was determined to have 7 planets.

Hydrus Constellation. Click to embiggen. 

However, Mike Tuomi of the University of Hertfordshire has re-examined the data collected by the HARPS Spectrograph (High Accuracy Radial Velocity Planet Searcher) and the 3.6m telescope at the La Silla Observatory in Chile; and by re-examining we mean the data was run through different statistical methods (Bayesian probability). In this way, it was determined that HD 10180 has 9 planets.

From the paper by Mike Tuomi:
"According to our results, there is evidence for up to nine planets orbiting HD 10180, which would make this star a record holder in having more planets in its orbits than there are in the Solar system. We revise the uncertainties of the previously reported 6 planets, verify the existence of the seventh signal, and announce the detection of two additional statistically significant signals in the data." 


There you have it, folks. A new King of the Galaxy has been crowned. All Hail the King!! Poor Solar system...now all its got is that whole life thing. Alas. Maybe we can promote Pluto back to its original posting (oh! oh! Put me in Coach!). Or better yet, perhaps we can consider the top 5 largest dwarf planets in our system full-fledged planets, making our total count 13 planets (do you know them? Ceres, Eris, Makemake, Huamea, and Pluto).

Mike Tuomi's Paper can be found here.
The Bad Astronomer's posting can be found here.
Io9 did a brief glance at this situation here.
Basic information about HD 10180 can be found here.
Astrobiology Magazine announcement can be found here.
Scientific American supports the new king.
Andrew Rushby's treatment of HD 10180.

A NASA animation of HD 10180