First Impression was “Moved and Relieved”

— That image taken in 2014 was widely featured by international media as a remarkable achievement of ALMA, and it also arouse much controversy among astronomers. What was your first impression when you saw the image?

Hasegawa: This image shows a planet forming disk around a young star called HL Tau, clearly revealing narrow concentric rings separated by gaps. It was much more detailed and more beautiful than we expected. I was moved and relieved at the same time.

Tetsuo Hasegawa (Senior Professor at the NAOJ Chile Observatory)

— Relieved? What kind of feeling was it?

Hasegawa: To make our budget request for ALMA, we explained our simulation to the government agency saying that ALMA is capable of taking more accurate astronomical images that have never been possible with existing radio telescopes, and also telling that ALMA will make great contributions to science. I was certain that ALMA would make it, but when I saw it in reality, honestly I was relieved (laugh).

Proposal Documents of ALMA Project. Computer simulation images depict the improvement of imaging capability of an interferometer at that time (left) and ALMA (right).　Credit：Geoff Bryden et al. （2000） ApJ, all rights reserved.

A Record-like Disk Reveals Formation of the Solar System

— To begin with, what is the star HL Tau like?

Hasegawa: It’s a young star approximately 450 light years from the Earth, merely a million years old.

— Our solar system is about 4.6 billion years old, right?

Hasegawa: Correct. In analogy, if we compare our 4.6 billion-year-old Sun to a 46-year-old man, HL Tau would be 0.01 years old, in other words only 4 days after birth.

— It’s like a baby star, rather than young. Does this whole orange disk consists of a baby star?

Hasegawa: No, the star is located at the center of this record-like disk. It is surrounded by gas and dust, which will grow into planets that orbit HL Tau. This clump of gas and dust is called “a protoplanetary disk”.

— Are planets formed in gas and dust?

Hasegawa: Right. Sun-like stars and planets like the Earth and Jupiter were formed in gas and dust floating in space. So, this image shows the very early stage of planet formation around a baby star.

— Did our solar system used to have a similar shape?

Hasegawa: If we could travel back to 4.6 billion years ago and see our solar system at the age of a million years, we would see a similar object like this.

ALMA Makes Blurry Image Incredibly Clear

— Is this the first image of a planet forming disk?

Hasegawa: The first reception of radio signals from a protoplanetary disk was made by the Nobeyama 45-m Telescope in 1993. And then later on, the Nobeyama Millimeter Array (NMA) captured this first image of a protoplanetary disk.

Radio spectrum from a gas disk　Credit：Skrutskie et al. 1993 ApJ 409, 422[1]. Reproduced by permission of the AAS, all rights reserved.

Radio spectrum from a gas disk around GG Tau　Credit：Kawabe et al. 1993, ApJ 404, 63[2]. Reproduced by permission of the AAS, all rights reserved.

— It doesn’t look like an astronomical object at all.

Hasegawa: A radio telescope has poor eyesight when compared with an optical telescope. In astronomical terms, we say, “low resolution”. The eyesight of a telescope increases in proportion to the aperture size, which means the eyesight improves with a larger size of lens or mirror for an optical telescope and with a larger size of antenna for a radio telescope. But, when we compare the eyesight between an optical and a radio telescope with the same size of aperture, the vision of a radio telescope is equivalent to only 1/10,000 of an optical telescope. Since the maximum extension of the NMA is up to 130 meters, it is only capable to take this level of blurred image. After this, various observations have been made and this is an image of HL Tau captured in 2002 by the NMA with higher resolution.

HL Tau observed with NMA　Credit：Kitamura et al. 2002, ApJ, 581, 357 [3] Reproduced by permission of the AAS, all rights reserved.

— Let’s see…hmm, this is just a line, isn’t it? It’s hard to believe this is the same object observed by ALMA this time.

Hasegawa: I agree with you. We can hardly imagine how planets will be formed from this image.

— But, by extending 66 antennas up to 16 km and combining received signals, we can make a virtual giant telescope ALMA that can clearly image this disk object in such details. Amazing!

Hasegawa: ALMA was constructed in global partnership of East Asia, North America, and the Member States of the European Southern Observatory. The total amount funded by Japan was about 250 million US dollars, which accounts for 1/4 of the entire construction costs. To secure such enormous amount of money for construction of ALMA from the national budget, efforts were needed to reduce research budgets allocated to other science projects not limited to the field of astronomy. So, to gain wider-ranging understanding of scientists, we gave explanations like, “This blurry image is the limit of our observation of planet formation at this point. However, if ALMA was constructed, we would be able to see it with 100 times higher resolution and reveal the planet formation process leading to the origin of life, which will be a great contribution not only to astronomy but to the science community as a whole. Could you give us support for it? ”

— This is why you were so relieved when you saw the first image.

Hasegawa: Exactly.

ALMA Telescope constructed in Chile Credit: Y. Beletsky (LCO)/ESO

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

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.

Our Solar System Proved to be “Not Normal”?

— What was the big discovery in 1995 that astounded astronomers?

Hasegawa: It was a discovery of an “extrasolar planet”. Planets that orbit stars other than the Sun are called extrasolar planets. In 1995, a group of researchers in Switzerland found that bright stars in the night sky have also a planet, similar to our solar system. So far, more than 3,500 planets have been found.

— Wow, over 3,000!

Artist’s conception of a giant gas planet that orbits close to the central star Credit: ESA – C. Carreau

Hasegawa: And, there have been found many “planetary systems” that have quite different shapes from that of our solar system. For example, 51 Pegasi, an extrasolar system found this time has a giant gas planet with a half the size of Jupiter that orbits close to the central star in only 4 days. Since Jupiter orbits far from the Sun in around 12 years, they are so different.

— It really overturned the conventional theory.

Hasegawa: Yes, it did. Until then, people believed that our solar system is a standard system as other planetary systems are. However, this discovery revealed that our solar system is only one of the variations of numerous planetary systems. Then attention is drawn to this question, “Why are there such variations in planetary systems?” To reach the answer, the key will be observing a protoplanetary disk where planets are forming.

— That would be the strong area of ALMA!!

Hasegawa: Right. ALMA is not designed to exclusively target planetary formation, but coincidentally an extrasolar planet was found during the construction of ALMA and it deepened discussions on planetary systems in astronomical communities. This heighten the motivation of observing planet forming object with ALMA and many researchers started working on this field.

If we approach close to HL Tau by spacecraft?

— Now, with ALMA, we can see how a planetary system is forming. If we could travel to HL Tau by spacecraft, what does it look like?

Hasegawa: Hmm, it’s an interesting question. We cannot see directly HL Tau from the Earth because HL Tau is surrounded by thin gas and dust, which block visible light. But, if we could somehow reach it to a close distance, it would look like a bright shiny object as shown by this image. The surface of the disk would be also bright reflecting the light from the central baby star.

— Could we enter inside the disk?

Hasegawa: I think we could.

— Won’t we be hurt by flying meteorites?

Hasegawa: Inside the disk, dust and gas are making rotational movement to a certain direction, but estimating from the scale of a disk like HL Tau, the size of dust would be only 1 mm in diameter. We can see where it comes from, so maybe we’re safe.

— Could you describe the inside of the disk?

Hasegawa: Supposedly it would be dark because dust concentrated into the disk will block the light from the central baby star.

— How about the temperature?

Hasegawa: It depends on the region. Since the temperature of a baby star is higher than an adult star, the place where the temperature is equivalent to the Earth will be around the orbit of Mars in analogy to our solar system. The outer region gets much colder toward the outer edge.

Observed Image of HL Tau Received Like a “Letter of Challenge” to Astronomers

— How will this planetary system evolve in near future?

Hasegawa: The central star is still a baby, it is supposed to grow into a star that has a size equivalent to the Sun.
 

— How about the planets around it?

Hasegawa: There are two possibilities: forming planets will gradually grow; or planets may disappear.

— s it possible that planets will disappear?

Hasegawa: Possible. For example, it is assumed that by the gravity between the planets and gas of the disk, the planets will be pulled to the central star and finally absorbed into it. In other cases, when a solar system contains more than 3 massive Jupiter-like planets, the orbits become unstable by the gravity of these planets and some of them may spin out the planetary system.

Artist’s conception of Kepler-16b with two suns Credit：NASA/JPL-Caltech/R. Hurt

— That’s shocking..

Hasegawa: According to the latest study, it is thought that the heavier the protoplanetary disk becomes, the more massive planets could be formed in a large number. I told earlier in this talk that our solar system used to be similar to the shape of HL Tau, but actually the disk of HL Tau is considered heavier than our solar system at the same age. Then, it might have a different shape from our solar system in future.

— You mean, further research is required?

Hasegawa: Yes. Our future expectation is to see a forming planet itself. At this moment, we regard the gaps inside the disk as a signature of forming planets, but if we make the best of the ALMA’s capability, we would be able to see a forming planet itself or gas movement in a disk that is being absorbed into planets.

— How interesting!!

Hasegawa: Furthermore, it is also expected that analysis will be made to know what kind of materials are contained in the disk using observing technology of ALMA. Especially if we could identify the quantity of molecules as the building blocks of life, it would be a very important observation result from the viewpoint of astrobiology.

— Final question. You said, it was not assumed that planets are being formed around such a young star as HL Tau. How will this issue be solved?

Hasegawa: As the research will advance by denying the past, researchers feel motivated by a discovery that makes them puzzled like this image observed by ALMA. Then, now we are having hard time in analyzing what we obtained, but it will lead us to a new paradigm of planet formation after 10 years of research. This image gave us a start.

— You mean, researchers want to be puzzled?

Hasegawa: You put it correctly. This image is a letter of challenge from the nature. Our mission as researchers is to find the answer to the challenge and rewrite our familiar text books.

At the NAOJ (Mitaka, Tokyo)

References

• [1] M. F. Skrutskie et al. “Detection of circumstellar gas associated with GG Tauri”, The Astrophysical Journal, vol. 409, page 422-428. Publication in May 1993.
• [2] R. Kawabe et al. “Discovery of a rotating protoplanetary gas disk around the young star GG Tauri”, The Astrophysical Journal Letters, vol.404, page L63-L66. Publication in February 1993.
• [3] Y. Kitamura et al. “Investigation of the Physical Properties of Protoplanetary Disks around T Tauri Stars by a 1 Arcsecond Imaging Survey: Evolution and Diversity of the Disks in Their Accretion Stage”, The Astrophysical Journal, vol.581, page 357-380. Publication in December 2002.