Research finds planets can form around different types of stars

New research shows that planets up to four times the size of the Earth can form around very different stars — including stars that are poorer in heavy elements. Credit: MediaFarm / Niels Bohr Institute

It had previously been thought that planets were more likely to form around a star if the star had a high content of heavier elements. But new research from the University of Copenhagen, among others, shows that small planets can form around very different types of stars – also stars that are relatively poor in heavy elements. This significantly increases the likelihood that Earth-like planets are widespread in the universe. The results have been published in the prestigious scientific journal, Nature.

3,000 exoplanets, i.e. planets orbiting a star other than the Sun, have now been discovered. 2,300 of these potential planets are being observed with the Kepler Satellite by measuring the brightness of the host . If a planet moves in front of its star, there is a small decrease in the brightness and if this happens repeatedly, it could be a planet orbiting the star and dimming its light.

A multitude of planets have been discovered so far and by measuring their size it is possible to distinguish between gas giants like Saturn and Jupiter, or whether they are smaller, terrestrial planets like Earth and Mars.

Link to Video: Astrophysicist Lars Buchhave, University of Copenhagen explains about his new research showing, that  planets up to four times the size of the Earth can form around very different stars — including stars that are poorer in heavy elements. The conclusion, says Lars A. Buchhave, is that these observations mean that Earth-like planets could be widespread throughout our galaxy, as they have no special requirements for an elevated content of heavy elements in stars in order to be formed. Credit: Niels Bohr Institute, University of Copenhagen

Requirements for planet formation?

But it is not only the planets that are interesting. It is also the stars that they are orbiting. Because what are the requirements for planet formation?

“I wanted to investigate whether planets only form around certain types of stars and whether there is a correlation between the size of the planets and the type of host star it is orbiting,” explains Lars A. Buchhave, astrophysicist at the Niels Bohr Institute and the interdisciplinary research centre, StarPlan at the University of Copenhagen.

Lars A. Buchhave therefore developed a method to ‘wring’ more information from the stellar spectra. Up until now, we have seen that most of the gas giants were associated with stars with a high content of heavy . For a star to have a high content of heavy elements it has to have gone through a series of rebirths.

Cosmic cycle

A star is a large ball of glowing gas that produces energy by fusing hydrogen and helium into heavier and heavier elements. When the entire core has been converted into iron, no more energy can be extracted and the star dies flinging massive clouds of dust and gas out into space. These large clouds of gas and dust condense and are recycled into new stars and planets in a gigantic cosmic cycle. The new stars that are formed will have a higher content of than the previous and for each generation of star formation there are more and more of the heavy elements and metals.

Remnants from the stars

The planets are formed from the remnants of the clouds of gas and dust that rotate in disc around the newly formed star. In this protoplanetary disc, the elements begin to accumulate and clump together and slowly the planets are formed.

In the later generations of stars with a high content of heavy elements, the rotating disc of dust and gas particles has an elemental composition that is most likely to promote the formation of gas giants like Saturn and Jupiter.

Recent research shows a different picture for the smaller planets.

Fewer requirements for small planets

“We have analysed the spectroscopic elemental composition of the stars for 226 exoplanets. Most of the planets are small, i.e. planets corresponding to the solid planets in our solar system or up to four times the Earth’s radius. What we have discovered is that, unlike the , the occurrence of smaller planets is not strongly dependent on stars with a high content of heavy elements. Planets that are up to four times the size of Earth can form around very different stars – also stars that are poorer in heavy elements,” Lars A. Buchhave.

The conclusion, says Lars A. Buchhave, is that these observations mean that Earth-like planets could be widespread throughout our galaxy, as they have no special requirements for an elevated content of heavy elements in stars in order to be formed. This conclusion resonates well with the picture that is emerging of the distribution of small planets in our galaxy, namely that it seems more the rule than the exception that a star has small planets orbiting them.

Because small Earth-like are not dependent upon a high content of in their host star, they could be both widespread and could have been formed earlier in our galaxy.

Alien earths could form earlier than expected

Building a terrestrial planet requires raw materials that weren’t available in the early history of the universe. The Big Bang filled space with hydrogen and helium. Chemical elements like silicon and oxygen – key components of rocks – had to be cooked up over time by stars. But how long did that take? How many of such heavy elements do you need to form planets?

Previous studies have shown that Jupiter-sized tend to form around stars containing more heavy elements than the Sun. However, new research by a team of astronomers found that planets smaller than Neptune are located around a wide variety of stars, including those with fewer heavy elements than the Sun. As a result, rocky worlds like Earth could have formed earlier than expected in the universe’s history.

“This work suggests that terrestrial worlds could form at almost any time in our galaxy’s history,” said Smithsonian astronomer David Latham (Harvard-Smithsonian Center for Astrophysics). “You don’t need many earlier generations of stars.”

Latham played a lead role in the study, which was led by Lars A. Buchhave from the University of Copenhagen and will be published in the journal Nature. The work is being presented today in a press conference at the 220th meeting of the .

Astronomers call heavier than hydrogen and “metals.” They measure the metal content, or metallicities, of other stars using the Sun as a benchmark. Stars with more heavy elements are considered metal-rich while stars with fewer heavy elements are considered metal-poor.

Latham and his colleagues examined more than 150 stars known to have planets, based on data from NASA’s spacecraft. They measured the stars’ metallicities and correlated that with the sizes of the associated planets. Large planets tended to orbit stars with solar metallicities or higher. Smaller worlds, though, were found around metal-rich and metal-poor stars alike.

prefer metal-rich stars. Little ones don’t,” explained Latham.

They found that terrestrial planets form at a wide range of metallicities, including systems with only one-quarter of the Sun’s metal content.

Their discovery supports the “core accretion” model of planet formation. In this model, primordial dust accumulates into mile-sized planetesimals that then coalesce into full-fledged planets. The largest, weighing 10 times Earth, can then gather surrounding hydrogen and become a gas giant.

A gas giant’s core must form quickly since hydrogen in the protoplanetary disk dissipates rapidly, swept away by stellar winds in just a few million years. Higher metallicities might support the formation of large cores, explaining why we’re more likely to find a gas giant orbiting a metal-rich star.

“This result fits with the core accretion model of planet formation in a natural way,” said Latham.

Journal reference:Nature

Provided byHarvard-Smithsonian Center for Astrophysics

More information: http://www.nature. … re11121.html

Journal reference:Nature

Provided byUniversity of Copenhagen

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