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A treasure trove of planets

eso1035a.jpgThere may be no place like home but the solar system around HD 10180 seems to be getting closer.

Astronomers at the European Southern Observatory (ESO) now say this 4.3 billion-year-old, sun-like star has at least five planets circling around it, a similar number to our own eight-planet solar system. Furthermore, there is evidence for two more planets in the system, one of which has a mass around 1.4 times that of the Earth, which would make it the smallest exoplanet found to date.

Unfortunately, the similarity ends there. The Earth-sized candidate orbits closest in, completing its “year” in 1.18 days. If it really exists, searing surface temperatures would make it an unlikely candidate for life. The five confirmed planets, all roughly Neptune-sized, are also relatively close, with orbits smaller than Mars’ path around the Sun. The other unconfirmed candidate would have a mass roughly equal to Saturn and orbit in 2200 days, approximately half of Jupiter’s year.


Researchers spotted this lavish system, located 127 light years away in the southern constellation Hydrus, by looking for a characteristic wobble in the parent star. As the orbiting planets gravitationally tug on their host, they induce this movement, which astronomers see as a complex back-and-forth motion. Fifteen years ago, the same method let to the first confirmed detection of an exoplanet around a sunlike star. With improved technology the approach is increasingly turning in a broader assortment of finds, including multiple planet systems such as this.

With this discovery, there are now 15 known stars with more than three planets. The previous record holder, 55 Cancri, has five confirmed planets orbiting it.

Strangely, the planets in this latest system also seem to adhere to their own version of the Titus-Bode law, an 18th century observation that each planet’s orbit is about twice as large as the preceding planet. In more recent times, the law was deemed to be a coincidence as it does not hold true for Neptune and lacks a theoretical basis. Perhaps the new system will reopen avenues of research into this historical curiosity.

You can view animation of the new system here.

Image: ESO/L. Calçada

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