Our history is written in the stars

The discovery of a cloud of the substance aluminium monoxide (AlO) around a distant young star confirms important details about how our solar system — and us — came to be.

The white inclusions on this chondrite meteorite are called CAIs. Rich in calcium and aluminium they are among the oldest solid matter in the solar system (© 2019 Rohan Mehra — Division for Strategic Public Relations)

The cloud of aluminium monoxide (AIO) was discovered by astronomers around the young star — Origin Source I — found roughly 1.4k light-years away from Earth in the Orion Nebula. The cloud’s limited distribution suggests that AlO gas rapidly condenses to solid grains, hinting what the early stages of our solar evolution could have looked like.

For Professor Shogo Tachibana of the UTokyo Organization for Planetary and Space Science space exploration is a passion. He examines things everything from meteorites to stars and nebulae — huge clouds of gas and dust in space — in his quest to uncover our solar system’s origins.

He says: “I have always wondered about the evolution of our solar system, of what must have taken place all those billions of years ago.

“This question leads me to investigate the physics and chemistry of asteroids and meteorites.”

Asteroids and meteorites — the solar system’s record in rocks

Asteroids and meteorites hold a deep fascination for astronomers as these rocks can remain largely unchanged since the time our Sun and planets formed from a swirling cloud of gas and dust. Potentially containing a record of the conditions at that time — roughly 4.56 billion years ago —their properties and composition can tell us a great deal about these early conditions.

Professor Shogo Tachibana UTokyo Organization for Planetary and Space Science (© 2019 Rohan Mehra — Division for Strategic Public Relations)

Tachibana explains: “On my desk is a small piece of the Allende meteorite, which fell to Earth in 1969. It’s mostly dark but there are some scattered white inclusions (foreign bodies enclosed in the rock), and these are important.

“These speckles are calcium and aluminium-rich inclusions (CAIs), which were the first solid objects formed in our solar system.”

A small chondrite meteorite containing minerals rich in calcium and aluminium (© 2019 Rohan Mehra — Division for Strategic Public Relations)

Minerals present in CAIs don’t just indicate that our young solar system must have been extremely hot. Physical techniques for dating these minerals reveal a fairly specific age for the solar system. Tachibana and colleagues want to expand on the details of this stage of stellar and planetary evolution.

For Tachibana this meant viewing a young at that same point in its lifecycle: “There are no time machines to explore our own past, so we wanted to see a young star that could share traits with our own.

“With the Atacama Large Millimeter/submillimeter Array (ALMA), we found the emission lines — a chemical fingerprint — for AlO in outflows from the circumstellar disc — the gas and dust surrounding a star — of the massive young star candidate Orion Source I.”

Whilst not exactly like our Sun, Tachibana adds that Orion Source I was a good start to such an investigation.

ALMA image showing AIO around the star Origin Source I at wavelengths 497 GHz (left) and 650 GHz (right) (© 2019 Astrophysical Journal Letters/ Shogo Tachibana)

And the Atacama Large Millimeter/submillimeter Array (ALMA) was the perfect tool for the observations. ALMA offers extremely high resolution and sensitivity— enough to reveal the distribution of AlO around the star — a feat no other current instrument could perform.

Tachibana explains: “Thanks to ALMA, we discovered the distribution of AlO around a young star for the first time. The distribution of AlO is limited to the hot region of the outflow from the disk. This implies that AlO rapidly condenses as solid grains — similar to CAIs in our solar system.

“This data allows us to place tighter constraints on hypotheses that describe our own stellar evolution. But there’s still much work to do.”

The team now plans to explore gas and solid molecules around other stars to gather data useful to further refine solar system models.

Original research: Shogo Tachibana, Takafumi Kamizuka, Tomoya Hirota, Nami Sakai, Yoko Oya, Aki Takigawa, and Satoshi Yamamoto, “Spatial distribution of AlO in a high mass protostar candidate Orion Source I,” Astrophysical Journal Letters: April 24, 2019, doi:10.3847/2041–8213/ab1653.