It's Over Your Head (2006)

060524.doc HD 69830: A Trio of Neptune-mass Exoplanets Page 1 of 5

From Celestial North, this is IT’S OVER YOUR HEAD for the week of May

24th, 2006, a look at what’s up in the sky over Puget Sound.

Barely a decade ago, the only planets we knew about were the nine in our own solar system. Since then, the list of extrasolar planets – exoplanets, for short – has exploded, with the count now standing at 182 – twenty times the number in our own Sun’s retinue. Most of these exoplanets are the size of

Jupiter or larger – hundreds of times more massive than Earth.

But now astronomers have hit a sort of exoplanet search trifecta by finding three Neptune-mass planets around a single nearby star. This trio of new exoplanets have minimum masses ranging between 10 and 18 times that of the

Earth.

The parent sun, HD 69830, is located about 41 light years away in the constellation Puppis, the stern of the now extinct constellation of Argo Navis. You may recall from Greek mythology that Jason and the Argonauts sailed aboard the

Argo in quest of the Golden Fleece. Shining at magnitude 5.95, HD 69830 is barely at the limit of naked eye visibility under very dark skies. Due to the foreshortened nights of late-spring and Puppis’ location low on the southwest horizon, this sun-like star is not currently well-placed for viewing. Late winter and early spring are better times to try to get a glimpse of it.

Though other Neptune-mass exoplanets have been discovered, this is the

first system encountered that has several worlds of this type. The innermost of

the three orbits its primary in under 9 days. For comparison, it takes Mercury,

the closest planet to our own Sun, nearly ten times longer to complete one orbit –

88 days to be precise. Being in such close proximity to its star, HD 69830’s

innermost Neptune-mass planet is likely rocky – similar to our own solar system’s

four inner terrestrial planets – though it is at least 10 times more massive than

Earth, and scorchingly hot.

The middle of the three planets has a 31.6-day orbital period, or about one

Earth month. Even though HD 69830 is only two-thirds as luminous as our Sun, you can imagine that this middle planet is still quite “toasty”, and probably has a rocky core enveloped by a gaseous atmosphere. At a minimum, this middle exoplanet has a mass equivalent to at least a dozen Earths.

The outermost Neptune-mass planet in this newly discovered system takes

197 days to complete one circuit around its parent star. That’s pretty close to

Venus’ 225-day orbit around the Sun. In this somewhat cooler region, this third planet may have been able to accrete some ice during its formation, leading to a rocky/icy core surrounded by what is likely a fairly massive atmosphere. The largest of the group, this one masses in at at least 18 Earths.

This outermost planet also apparently lies close to the inner edge of HD

69830’s habitable zone. Also informally called the “Goldilocks” zone – because

it’s neither so hot as to vaporize water, nor so cold as to freeze it – the habitable zone is the area around a star where the temperature is “just right” for water to exist in liquid form. This is always notable, since liquid water is thought to be one of the necessary ingredients for life. While the outermost of this Neptune-mass trio of exoplanets is almost certainly far from Earth-like due to it’s massive size and potentially crushing atmosphere, it still gets us much closer to the ultimate goal of finding Earth-size planets orbiting in the habitable zones around other stars.

Besides having multiple planets, and ones that are not so much larger than

Earth itself, this new extra-solar system apparently shares yet another similarity with our own solar system. Recent infrared observations of HD 69830 by the

Spitzer Space Telescope indicate this star may host an asteroid belt as well.

Discovering this triplet of Neptune-mass planets was no easy feat. Over the course of more than two years, astronomers observed minute Doppler shifts in the light from HD 69830 to tease out evidence of its planetary entourage.

They used the HARPS spectrograph on the European Space Agency’s 3.6-meter telescope atop La Silla, a 7800-foot mountain peak on the southern extremities of

Chile’s Atacama Desert. The thin, dust-free atmosphere at La Silla, combined with the absence of any significant sources of light pollution, make HARPS the world’s premier exoplanet hunting machine.

As a planet travels in its orbit, its relatively minute gravity tugs a little on its

parent star. When that tug pulls the star more in our direction, the light waves

from the star shift ever so slightly toward the blue end of the spectrum.

Conversely, when the planet’s tug pulls the star slightly away from us, the light

waves shift a smidgeon the other way – toward the red end of the spectrum.

This is similar to the change in pitch we hear when a rapidly moving train or

ambulance approaches us and then recedes into the distance. In the case of HD

69830’s planets, these tugs amounted to only 2-3 meters per second, equivalent to a brisk walking pace. And those tiny shifts were detected from objects 240 trillion – with a “T” – miles away!

We’re on the web at CelestialNorth.org. Until next time, this is ______and ______, with a reminder that the night is large and full of wonders.

REFERENCES: 1. http://spaceflightnow.com/news/n0605/18neptunes/ 2. http://www.spacedaily.com/reports/Astronomers_Catch_Planet_By_Unusu al_Means.html 3. http://www.princeton.edu/~willman/planetary_systems/HD69830.html 4. http://www.princeton.edu/~willman/planetary_systems/ 5. http://www.ls.eso.org/index.html