Universe Observed through Visual Acuity of 120,000/20 [vol.1] Astronomers Stunned by HL Tauri
Interviewed and written by Toshihiro Nakamura
Photographed by: Nozomu Toyoshima
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!
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.
— 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.
-  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.
-  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.
-  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.