First Exoplanet found around solar twin in star cluster

This artist's impression shows one of the three newly discovered planets in the star cluster Messier 67. 

In this cluster the stars are all about the same age and composition as the Sun. 

This makes it a perfect laboratory to study how many planets form in such a crowded environment. 

Very few planets in clusters are known and this one has the additional distinction of orbiting a solar twin -- a star that is almost identical to the Sun in all respects. Credit: ESO/L. Calçada

ESO HARPS detector cryostat

Astronomers have used ESO's HARPS planet hunter in Chile, along with other telescopes around the world, to discover three planets orbiting stars in the cluster Messier 67.

Although more than one thousand planets outside the Solar System are now confirmed, only a handful have been found in star clusters.

Remarkably one of these new exoplanets is orbiting a star that is a rare solar twin—a star that is almost identical to the Sun in all respects.

Planets orbiting stars outside the Solar System are now known to be very common.

These exoplanets have been found orbiting stars of widely varied ages and chemical compositions and are scattered across the sky but, up to now, very few planets have been found inside star clusters.

This is particularly odd as it is known that most stars are born in such clusters. Astronomers have wondered if there might be something different about planet formation in star clusters to explain this strange paucity.

Anna Brucalassi

Anna Brucalassi (Max Planck Institute for Extraterrestrial Physics, Garching, Germany), lead author of the new study, and her team wanted to find out more.

"In the Messier 67 star cluster the stars are all about the same age and composition as the Sun. This makes it a perfect laboratory to study how many planets form in such a crowded environment, and whether they form mostly around more massive or less massive stars."

The team used the HARPS planet-finding instrument on ESO's 3.6-metre telescope at the La Silla Observatory.

These results were supplemented with observations from several other observatories around the world.

They carefully monitored 88 selected stars in Messier 67 over a period of six years to look for the tiny telltale motions of the stars towards and away from Earth that reveal the presence of orbiting planets.

The small faint constellation Cancer is rich in open clusters and double stars. 

Credit: Starry Night Software, Space.com

This cluster lies about 2500 light-years away in the constellation of Cancer (The Crab) and contains about 500 stars.

Many of the cluster stars are fainter than those normally targeted for exoplanet searches and trying to detect the weak signal from possible planets pushed HARPS to the limit.

Three planets were discovered, two orbiting stars similar to the Sun and one orbiting a more massive and evolved red giant star.

The first two planets both have about one third the mass of Jupiter and orbit their host stars in seven and five days respectively. The third planet takes 122 days to orbit its host and is more massive than Jupiter.

The first of these planets proved to be orbiting a remarkable star—it is one of the most similar solar twins identified so far and is almost identical to the Sun.

It is the first solar twin in a cluster that has been found to have a planet.

Two of the three planets are "hot Jupiters"—planets comparable to Jupiter in size, but much closer to their parent stars and hence much hotter.

All three are closer to their host stars than the habitable zone where liquid water could exist.

Luca Pasquini

"These new results show that planets in open star clusters are about as common as they are around isolated stars—but they are not easy to detect," adds Luca Pasquini (ESO, Garching, Germany), co-author of the new paper.

"The new results are in contrast to earlier work that failed to find cluster planets, but agrees with some other more recent observations. We are continuing to observe this cluster to find how stars with and without planets differ in mass and chemical makeup."

More information: This research was presented in a paper entitled "Three planetary companions around M67 stars", by A. Brucalassi et al., to appear in the journal Astronomy & Astrophysics. (PDF)

ESA VLT: Oldest solar twin identified

This image tracks the life of a Sun-like star, from its birth on the left side of the frame to its evolution into a red giant star on the right. 

On the left the star is seen as a protostar, embedded within a dusty disc of material as it forms. It later becomes a star like our Sun. 

After spending the majority of its life in this stage, the star's core begins to gradually heat up, the star expands and becomes redder until it transforms into a red giant. 

Following this stage, the star will push its outer layers into the surrounding space to form an object known as a planetary nebula, while the core of the star itself will cool into a small, dense remnant called a white dwarf star. 

Marked on the lower timeline are where our Sun and solar twins 18 Sco and HIP 102152 are in this life cycle. 

The Sun is 4.6 billion years old and 18 Sco is 2.9 billion years old, while the oldest solar twin is some 8.2 billion years old -- the oldest solar twin ever identified. 

By studying HIP 102152, we can get a glimpse of what the future holds for our Sun. 

This image is illustrative; the ages, sizes, and colours are approximate (not to scale). The protostar stage, on the far left of this image, can be some 2000 times larger than our Sun. 

The red giant stage, on the far right of this image, can be some 100 times larger than the Sun. 

Credit: ESO/M. Kornmesser

An international team led by astronomers in Brazil has used ESO's Very Large Telescope to identify and study the oldest solar twin known to date.

Located 250 light-years from Earth, the star HIP 102152 is more like the Sun than any other solar twin—except that it is nearly four billion years older.

This older, but almost identical, twin gives us an unprecedented chance to see how the Sun will look when it ages.

The new observations also provide an important first clear link between a star's age and its lithium content, and in addition suggest that HIP 102152 may be host to rocky terrestrial planets.

Astronomers have only been observing the Sun with telescopes for 400 years—a tiny fraction of the Sun's age of 4.6 billion years.

It is very hard to study the history and future evolution of our star, but we can do this by hunting for rare stars that are almost exactly like our own, but at different stages of their lives.

Now astronomers have identified a star that is essentially an identical twin to our Sun, but 4 billion years older—almost like seeing a real version of the twin paradox in action.

Jorge Melendez (Universidade de São Paulo, Brazil), the leader of the team and co-author of the new paper explains: "For decades, astronomers have been searching for solar twins in order to know our own life-giving Sun better.

But very few have been found since the first one was discovered in 1997. We have now obtained superb-quality spectra from the VLT and can scrutinise solar twins with extreme precision, to answer the question of whether the Sun is special."

The team studied two solar twins—one that was thought to be younger than the Sun (18 Scorpii) and one that was expected to be older (HIP 102152).

They used the UVES spectrograph on the Very Large Telescope (VLT) at ESO's Paranal Observatory to split up the light into its component colours so that the chemical composition and other properties of these stars could be studied in great detail.

They found that HIP 102152 in the constellation of Capricornus (The Sea Goat) is the oldest solar twin known to date.

It is estimated to be 8.2 billion years old, compared to 4.6 billion years for our own Sun. On the other hand 18 Scorpii was confirmed to be younger than the Sun—about 2.9 billion years old.

More information: This research was presented in a paper to appear in "High precision abundances of the old solar twin HIP 102152: insights on Li depletion from the oldest Sun", by TalaWanda Monroe et al. in the Astrophysical Journal Letters. Research paper PDF