HD 10180 is a star that’s nearly the Sun’s twin: it’s very close in mass, temperature, brightness, and even chemical content of our friendly neighborhood star. But in this case of stellar sibling rivalry, HD 10180 may have the upper hand: a new analysis of observations of the star indicate it may have nine planets!
In a new report accepted for publication in the journal Astronomy and Astrophysics, an astronomer re-analyzed data of the star taken with the High Accuracy Radial Velocity Planet
Searcher (HARPS), an exquisitely high-precision camera mounted on a 3.6 meter telescope in Chile. HARPS has been observing HD 10180 for years; the star is a mere 130 light years away, making it bright and easy to study. The observations look to see if the star exhibits a periodic shift in its light: a Doppler shift as planets circle it, tugging it one way and another.
Six clear Doppler shift signals were found in the original analysis: six planets, five of which have masses ranging from 12 – 25 times that of the Earth (making them more like Neptune than our own comfortable planet), and a sixth that was bigger yet, 65 times Earth’s mass (more like Saturn than Neptune). These planets orbit HD 10180 with periods of 5 – 2000 days. A seventh possible planet was detected, but the data weren’t strong enough to make a solid claim.
The new analysis looks at the old data in a different way, examining it using different statistical methods. Not only are the six planets seen in the new results, but the seventh is confirmed, as well as finding two additional planets in the data. If this result pans out, that means HD 10180 has nine planets, more than our solar system does!
The three additional planets have masses of 1.3, 1.9, and 5.1 times that of Earth, and orbit the star with periods (think of that as the planets’ years) of 1.2, 10, and 68 days, respectively.
Those first two are pretty firmly in the Earth-mass range, what astronomers call "super Earths". However, Earth-like they ain’t: they’d be cooked by the star. The first is only 3 million km (less than 2 million miles) from HD 10180, and the second barely any cooler at about 14 million km (8 million miles). This is much closer to the star than Mercury is to the Sun, and remember HD 10180 is very much like the Sun. If those planets are rocky, their surfaces are hot enough to melt tin, zinc, and on that inner planet, maybe even iron.
So yeah, not exactly a fun place to visit.
Astronomers have announced a whole new passel of planets orbiting stars near the Sun: 50 more, the single largest group ever announced at one time*. It’s an indication of just how good we’re getting at finding these things. Even better: many of these planets are at the upper end of what we might call Earth-like.
ESO artist drawing of an Earth-like planet orbiting a Sun-like star. Click to enplanetate. Credit: ESO/M. Kornmesser
The instrument, called HARPS, for High Accuracy Radial velocity Planet Searcher, is a spectrograph. It takes light from distant objects and breaks it down into incredibly thin slices of wavelength, like a rainbow with a hundred thousand colors†. A planet orbiting a star tugs on the star with its gravity, pulling it toward and away from us over the course of its orbit. This makes an incredibly small shift in the colors of the star, but well within the capability of HARPS to detect.
The team used HARPS (PDF) to survey 376 stars like the Sun (similar mass, size, and temperature) and found that 40% of them have at least one planet with less mass than Saturn. 40%! That’s incredible! Just doing some very rough math, 10% of the stars in the Milky Way could easily be called Sun-like. If 40% of those host planets the size of Saturn or smaller, there are billions of planets that size in our galaxy alone.
That’s a whole lot of planets.
They also found that Neptune-mass planets appear to be common in multiple planet systems; in fact that’s apparently where the majority of them are. In other words, planets like Neptune are pretty easy to make, especially when there are other planets in the system.
16 of the 50 planets discovered by HARPS in this release are so-called "super-Earths", with masses a few times that of our home planet. Interestingly, you might expect them to have higher gravity than Earth since they’re more massive, but if they are also less dense — that is, bigger in size — then you might not weigh all that much more standing on the surface of one. For example, a planet with 5 times Earth’s mass but twice the radius would have a surface gravity only 20% higher than Earth; if you weighed 150 pounds here you’d weigh 180 pounds there.