Mini-Neptune found around star with lowest metallicity yet

December 29, 2015

Astronomers discovered a planet smaller than Neptune, so called mini-Neptune, around a star with low metallicity. While Jupiter-like planets need metal-rich stars to form, smaller planets have a lower metallicity threshold. Of all the stars with small planets orbiting them, HD175607 has the least metal yet. This will help us understand just how low metallicity can be for planets to still form.

Mini-Neptunes don't necessarily need metal-rich stars. Credit: NASA

With each new exoplanet discovered, we get a little bit closer to understanding if our own solar system is in any way special or if the number of planets, their positions and their sizes are quite common in our galaxy. What kind of planets will a star system have, depends on how many elements heavier than hydrogen and helium there are in a star. This is called metallicity of a star.

High star metallicity means forming planets had needed heavy elements

When our Universe was created, hydrogen and helium were almost the only two elements in it. From clouds of hydrogen and helium gas, the first stars were born. They had no planets orbiting them because planets need heavier elements to form. This first generation of stars created those heavier elements via fusion at their cores . 

Then, as they ended their lives, stars ejected these heavy elements back into space. Think supernova for big stars or simply shedding the outer layers for smaller stars. This created new clouds of gas and dust that new, metal-rich stars and planets are made of.

Now we’re going somewhere. Now there’s not only hydrogen and helium, but also heavier elements available for planet formation. Gas giants like Jupiter have a better chance of forming around stars with high metallicity. Heavier elements clump up faster, thus attracting more material in less time, creating planetesimals that then begin accreting gas. Jupiter-like giants are so big because they had enough time to attract gas and dust before the necessary material dissipated.

Mini-Neptunes don’t necessarily need metal-rich stars

For planets smaller than Jupiter, like mini-Neptunes with masses of about 10 times the mass of Earth, high metallicity doesn’t appear to be crucial and this latest discovery supports it.

A team of astronomers led by Annelies Mortier of the University of St. Andrews, UK, found a low metallicity star called HD175607 with mini-Neptune planet orbiting it. This particular mini-Neptune, called HD175607b, is 8 to 10 times heavier than Earth. The planet is orbiting the star at only one tenth of the distance between Earth and Sun and it takes it only 29 days for a single orbit.

Small planets are harder to detect, so exoplanet-searching efforts focus more on looking for gas giants around metal-rich stars. This makes correlation between star’s metallicity and small planet abundance unclear.

"For Neptunes and Earthlike planets, it is not as clear yet what the role of metallicity is," Mortier said.

Low metallicity stars are acceptable targets for searching exoplanets

HD175607 is a G dwarf star about 147 light-years from Earth weighing about 25% less than Sun. While its metallicity is too low for gas giants to form, its example shows that smaller planets can still exist around metal-poor stars. This means that the search for Earth-like planets around metal-poor dwarf stars makes sense.

For finding stars with low metallicity, the team used the High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph. For detecting the mini-Neptune orbiting the HD175607, they used the technique called radial velocity - detecting how planet’s gravitational pull affects the star. The study appears in Astronomy and Astrophysics journal.

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