This is the sharpest image ever taken by ALMA — sharper than is routinely achieved in visible light with the NASA/ESA Hubble Space Telescope. It shows the protoplanetary disc surrounding the young star HL Tauri. These new ALMA observations reveal substructures within the disc that have never been seen before and even show the possible positions of planets forming in the dark patches within the system.

Universe Observed through Visual Acuity of 120,000/20 [vol.1] Astronomers Stunned by HL Tauri

In 2014, astronomers were stunned by the ultra-high resolution image of HL Tauri (also called HL Tau) observed by ALMA, revealing a key element to unveil the formation process of a planetary system. What is so extraordinary about HL Tau that looks like a record floating in the night sky? For the answer, we interviewed with Professor Tetsuo Hasegawa at the NAOJ Chile Observatory.
Interviewed and written by Toshihiro Nakamura
Photographed by: Nozomu Toyoshima

Observed Gaps Far More Beautiful than CG Simulation

— ALMA has unveiled the early stage of the planet forming process into details. Could you tell me what was discovered with this image of HL Tau?

Hasegawa: Do you remember that stars and planets were formed in gas and dust floating in space? I will explain this further showing this figure.


Artist's conception of planet formation

Artist’s conception of planet formation

Hasegawa: Stars and planets are coming into shape while clumps of gas and dust floating in space shrink by its own gravity. When shrunk, the temperature and density of a clump get higher which strengthens the gravity and accelerates the contraction. In this way, gas and dust flow into the center of the clump and it creates rotational movement like water forms a whirlpool when draining water in a sink. As a result of this movement, gas and dust forms a shape like a rotational disk.


— This is how this shape was formed. Does a disk really rotate?

Hasegawa: Inside the disk, gas and dust rotate together into a direction around the baby star at the center of the disk. Gas and dust accumulated at the center increase the gravity and density of the core which generates energy to trigger nuclear fusion at 15 million degrees Celsius. After the nuclear fusion, the star becomes an adult star called a “main-sequence star”.


— There are several concentric dark gaps inside the disk. What are these gaps?

Hasegawa: This is the very discovery of ALMA. These gaps are considered as signatures of a growing large Jupiter-like planet that sweeps its orbits clear of debris. Gas and dust are pulled toward such a large-sized planet by gravity and make these gaps. In the simulation study of planet formation, these gaps were assumed to be formed as a hypothesis.



CG simulation of planet formation Credit:Bryden et al. (2000)ApJ

— Such prediction was proved true by ALMA.

Hasegawa: Right. And, more surprisingly, the observed object looks far more beautiful than CG. Natural creation is more refined and more graceful than human imagination, I thought.


After a Heartfelt Moment, a Big Question Comes…

— Gaps were observed as predicted, but aren’t there anything different from the simulation?

Hasegawa: Yes, there are some differences. I was looking at the image for a while, and then thought, “Wait, why so many gaps are there around the baby star?”


— Didn’t you say the gaps were predicted by simulation?




Hasegawa: Actually, it was believed that these gaps are formed in a disk when the central star is around 10 million years old.


— You mean, a million-year-old HL tau is too young to have gaps?

Hasegawa: Exactly. Gaps are considered as an evidence of a forming large planet. However, it wasn’t expected that so many large planets are growing around such a young star at the age of 1 million years in a “standard model” that was long accepted by astronomers. So, after the moment of relief and admiration of its beauty, a big question comes like “What on earth is this?”


— Could you explain what the standard model is?

Hasegawa: It’s a theory of planet formation explaining how the planets of our solar system were formed. It was originally proposed by a research group led by Chushiro Hayashi at Kyoto University who played a pioneering role in the field of Japanese astrophysics, and has been developed over time.



Artist’s conception of the standard model of planet formation theory

— Didn’t the standard model give the basis for the gaps in the disk of HL Tau.

Hasegawa: No. In the standard model, it was believed that planets like the Earth and Jupiter around the Sun would be formed after the central star of the solar system (the Sun) has grown mature to a certain phase. However, in the case of HL Tau, large planets have already stared to form even with an immature central star. In this regard, HL Tau is very different from what the conventional theory of planet formation suggested.


— Did such inconsistency with the conventional theory cause any confusion among astronomers?

Hasegawa: Actually, we had a similar big discovery in the planet research in 1995. With new findings, previous beliefs and theories on planets were overturned in a sense. Having gone through this big change, the research of planet formation has been a “hot spot” in astronomy and many astronomers are paying great attention to the ALMA image of the planet forming disk.


Tetsuo Hasegawa (Senior Professor at the NAOJ Chile Observatory)

Tetsuo Hasegawa (Senior Professor at the NAOJ Chile Observatory)

Born in Tochigi Prefecture in 1955. Doctor of Science in Astronomy, the University of Tokyo. After served as a research assistant at the Tokyo Astronomical Observatory of the University of Tokyo (the precursor of the National Astronomical Observatory of Japan) and then an assistant professor at the Institute of Astronomy, School of Science, the University of Tokyo, he was appointed to a professor at the National Astronomical Observatory. Engaged in the research of star and planet formation in the field of radio astronomy. Led the ALMA Project from the initial phase as NAOJ-ALMA Project Manager, as well as Deputy Project Manager of the Joint ALMA Observatory.