Wednesday, February 9, 2011

Pseudoscience vs Science

Akhir - akhir ini banyak sekali berita di media massa yang nampak seperti science namun sebenarnya hanya pseudo-science. Berikut ini ada artikel yang bagus mengenai hal ini, yang dikutip dari Selamat membaca dan semoga bermanfaat.

"Yes, the world would be a more interesting place if there were UFOs lurking in the deep waters off Bermuda and eating ships and planes, or if dead people could take control of our hands and write us messages. It would be fascinating if adolescents were able to make telephone handsets rocket off their cradles just be thinking at them, or if our dreams could, more often than can be explained by chance and our knowledge of the world, accurately foretell the future." Just one nice passage among many, many in Carl Sagan's "The Demon-Haunted World" (available in the campus library and most public libraries). Well, Dr. Sagan, if the world would be more interesting if the unexplained UFOs were in fact space aliens, if we could communicate with the dead or space aliens, etc., why are you scientists such stuffy, party-poopers, insisting that the evidence is not good enough to prove that these things exist? With thousands of eye-witnesses, what more do you need? Sagan wrote that passage above just before he discussed pseudoscience in "The Demon-Haunted World". If we understand the difference between real science and pseudoscience, perhaps we can understand the view of many scientists and skeptics that the UFO research is pseudoscience.

"Pseudo" means "not genuine; sham", something pretending to be something else that it is not. Pseudosciences "purport to use the methods and findings of science, while in fact they are faithless to its nature—often because they are based on insufficient evidence or because they ignore clues that point the other way" (Sagan, 1996). We are awash in pseudoscience from all around us because "pseudoscience is easier to contrive than science" ("contrive" is a pretty strong word choice by Dr. Sagan). With pseudoscience, the standards of argument and what is allowable as evidence are much more relaxed than what you find in science.

This is not to say that all of science is correct. No, there have been plenty of mistakes in science, plenty of blind alleys. No, reality is messier, more unpredictable than the best detective/murder-mystery novel. With science, hypotheses are framed in a way that they can be tested by experiment and observation. Nature has the final veto power in whatever explanation we come up with but scientists are human (yes, they are) and subject to emotional attachments to their explanations. They too can be offended when their pet explanation doesn't pan out, when Nature has vetoed it.

Pseudoscience is just the opposite. Hypotheses are often framed in a way that makes them untestable. "Practitioners [of pseudoscience] are defensive and wary. Skeptical scrutiny is opposed. When the pseudoscientific hypothesis fails to catch fire with scientists, conspiracies to suppress it are deduced" (Sagan, 1996). Ah, yes! How many times have we heard that the science journals won't publish the UFO research with charges of bias and close-mindedness on the part of the science "establishment"? Such charges are part of the conspiracy mindset. I'm sorry, but it is not a conspiracy. It is because the UFO evidence is not of the caliber needed to base conclusions upon and less fantastic alternative explanations that don't involve space aliens are not addressed or explored by the author of the proposed paper. Not every truly scientific paper makes it into the journals either but the scientist doesn't complain of a conspiracy. No, the paper was probably rejected because more data needed to be gathered to improve the signal (the confidence level) above the ever-present statistical fluctuations of reality in order to deduce the conclusion reached by the author. Sometimes, too strong a conclusion is deduced from too weak a data set. Another likelihood is that the author did not explore an alternative explanation because they failed to see the assumptions that they were operating under. Our filters can blind us to the obvious.

"Perhaps the sharpest distinction between science and pseudoscience is that science has a far keener appreciation of human imperfections and fallibility than does pseudoscience." (Sagan, 1996) This is why conclusions based solely (or even mostly) on eye-witness testimony are not acceptable in science, however harsh that may seem to the layman. The Innocence Project (see has shown that eyewitness identification has played a significant role in 75% of the convictions that were later overturned through DNA testing. Thirty years of social science research has proven that eyewitness identification is often unreliable. Even victims of horrendous personal crimes have mis-identified the perpetrators. Unfortunately, our memories are malleable. Initial uncertainties in recollection become strongly-held beliefs, bed-rock certainties, once we've had time to try to make sense of what happened. Our creativity can sometimes lead us astray. It can happen to the best of us. Even scientists. Please see Christopher Chabris' and Daniel Simons' Invisible Gorilla website for some of this research and especially see Dan Simons' "Counter-Intuition" talk he gave in April 2010. There is a video of his short talk in the video section of Invisible Gorilla in which he gives powerful examples of our perceptions, intuitions, and even the reasoning about our intuition leading even the best of observers astray. That is why scientists lay their results open to the very critical scrutiny of others. And they agree to accept the criticism and re-submit their work when they have improved their argument through better data or give it up when the observations show that their idea does not have merit. They don't blame the "establishment".

So, it is not because scientists just don't want to believe in space aliens that they are critical of the claims of UFOs as aliens, it is because time and time again the methodology of the UFO claims have not followed the high standards of verifiable scientific research. Has every claim of UFOs-as-space aliens been investigated? No. There are so many! It takes more time and energy to figure out the ordinary, natural cause of something than it takes for creative people to imagine fantastic things. Perhaps scientists are a bit too quick to discount UFOs-as-space aliens claims but after years of going down that dead end interpretation of noisy data, can you understand why they might want to devote their time to something more provable?

The next several pages are lengthy excerpts of Sagan's "The Demon-Haunted World: Science as a Candle in the Dark", published by Ballantine Books (New York) in 1996 (ISBN 0-345-40946-9). These excerpts are examples of alternative, plausible explanations that "point the other way" from that of space aliens and government cover-ups of space alien invasions.

Roswell, New Mexico

What follows is an excerpt from Sagan's "The Demon-Haunted World" about the alleged flying saucer crash in Roswell, NM in 1947 (page 84-86).

A great to-do has been made of one or more alleged crashed flying saucers near Roswell, New Mexico, in 1947. Some initial reports and newspaper photographs of the incident are entirely consistent with the idea that the debris was a crashed high-altitude balloon. But other residents of the region—especially decades later—remember more exotic materials, enigmatic hieroglyphics, threats by military personnel to witnesses if they didn't keep what they knew to themselves, and the canonical story that alien machinery and body parts were packed into an airplane and flown to the Air Materiel Command at Wright-Patterson Air force Base. Some, but not all, of the recovered alien body stories are associated with this incident.

Philip Class, a long-time and dedicated UFO skeptic, has uncovered a subsequently declassified letter dated July 27, 1948, a year after the Roswell" incident," from Major General C.B. Cabell—then Director of Intelligence for the U.S. Air Force (and later, as a CIA offical, a major figure in the abortive U.S. invasion of Cuba at the Bay of Pigs). Cabell was inquiring of those who reported to him on what UFOs might be. He hadn't a clue. In an October 11, 1948 summary response, explicitly including information in the possession of the Air Materiel Command, we find the Director of Intelligence being told that nobody else in the Air Force had a clue either. This makes it unlikely that UFO fragments and occupants had made their way to Wright-Patterson the year before.

What the Air Force was mostly worried about was that UFOs were Russian. Why Russians would be testing flying saucers over the United States was a puzzle to which the following four answers were proposed: "(1) To negate U.S. confidence in the atom bomb as the most advanced and decisive weapon in warfare. (2) To perform photographic reconnaissance missions. (3) To test U.S. air defenses. (4) To conduct familiarization flights [for strategic bombers] over U.S. territory." We now know that UFOs neither were or are Russian, and however dedicated the Soviet interest may have been to objectives (1) through (4), flying saucers weren't how they pursued these objectives.

Much of the evidence regarding the Roswell "incident" seems to point to a cluster of high-altitude classified balloons, perhaps launched from nearby Alamogordo Army Air Field or White Sands Proving Ground, that crashed near Roswell, the debris of secret instruments hurriedly collected by earnest military personnel, early press reports announcing that it was a spaceship from another planet ("RAAF Captures Flying Saucer on Ranch in Roswell Region"), diverse recollections simmering over the years, and memories refreshed by the opportunity for a little fame and fortune. (Two UFO museums in Roswell are leading tourist stops.)
A 1994 report ordered by the Secretary of the Air Force and the Department of Defense in response to prodding from a New Mexico Congressman identifies the Roswell debris as remnants of a long-range, highly secret, balloon-borne low-frequency acoustic detection system call "Project Mogul"—an attempt to sense Soviet nuclear weapons explosions at tropopause altitudes. The Air Force investigators, rummaging comprehensively through the secret files of 1947, found no evidence of heightened message traffic:

There were no indications and warnings, notice of alerts, or a higher tempo of operational activity reported that would be logically generated if an alien craft, whose intentions were unknown, entered U.S. territory…The records indicated that none of this happened (or if it did, it was controlled by a security system so efficient and tight that no one, U.S. or otherwise, has been able to duplicate it since. If such a system had been in effect at the time, it would have also been used to protect our atomic secrets from the Soviets, which history has shown obviously was not the case.)

The radar targets carried by the balloons were partly manufactured by novelty and toy companies in New York, whose inventory of decorative icons seems to have been remembered many years later as alien hieroglyphics.

In an earlier passage Sagan notes that balloons were extensively used by the Air Force in the 1950s for various uses including robotic espionage craft, with high-resolution cameras and signal intelligence devices. "High-altitude balloons can seem saucer-shaped when seen from the ground. If you misestimate how far away they are, you can easily imagine them going absurdly fast. Occasionally, propelled by a gust of wind, they make abrupt changes in direction, uncharacteristic of aircraft and in seeming defiance of the conservation of momentum—if you don't realize that they're hollow and weigh almost nothing." (p. 83) Please remember this when you read about reports of alien craft capable of accelerations and sudden changes of trajectory that are impossible with modern aircraft and would create fatal g-forces for humans.


Another excerpt from Sagan's "The Demon-Haunted World" that gives a plausible explanation of the unknown radar events during the Cold War that were kept under wraps (p. 86-87):

Consider spoofing. In the strategic confrontation between the United states and the Soviet Union, the adequacy of air defenses was a vital issue. It was item (3) on General Cabell's list. If you could find a weakness, it might be the key to "victory" in an all-out nuclear war. The only sure way to test your adversary's defenses is to fly an aircraft over their borders and see how long it takes for them to notice. The United States did this routinely to test Soviet air defenses.

In the 1950s and ‘60s, the United States had state-of-the-art radar defense systems covering its west and east coats, and especially its northern approaches (over which a Soviet bomber or missile attack would most likely come). But there was a soft underbelly—no significant early warning system to detect the geographically much more taxing southern approach. This is of course information vital for a potential adversary. It immediately suggests a spoof: One or more of the adversary's high-performance aircraft zoom out of the Caribbean, let's say, into U.S. airspace, penetrating, let's say, a few hundred miles up the Mississippi River until a U.S. air defense radar locks on. Then the intruders hightail it out of there. (Or, as a control experiment, a unit of U.S. high-performance aircraft is sequestered and sent in unannounced sorties to determine how porous American air defenses are.) In such a case, there may be combined visual and radar sightings by military and civilian observers and large numbers of independent reports. What is reported corresponds to no known aircraft. The Air Force and civilian aviation authorities truthfully state that none their aircraft was responsible. Even if they've been urging Congress to fund a southern Early Warning System, the Air Force is unlikely to admit that Soviet or Cuban aircraft got to New Orleans, much less Memphis, before anybody caught on.

Here again, we have every reason to expect a high-level technical investigating team, Air Force and civilian observers told to keep their mouths shut, and not just the appearance but the reality of suppression of data. Again, this conspiracy of silence need have nothing to do with alien spacecraft. Even decades later, there are bureaucratic reasons for the Department of Defense to be close-mouthed about such embarrassments. There is a potential conflict of interest between parochial concerns of the Department of Defense and the solution of the UFO enigma.

Government Conspiracies

One last excerpt from Sagan's "The Demon-Haunted World" about oft-lodged charge of the government's conspiracy of silence (p. 92-93). (Any more excerpts and I'll surely be charged with copyright infringement—please read the book for more! Though much lengthier excerpts are available for free on the Google Books version...)

A cover-up to keep knowledge of extraterrestrial life or alien abductions almost wholly secret for 45 years, with hundreds if not thousands of government employees privy to it, is a remarkable notion. Certainly, government secrets are routinely kept, even secrets of substantial general interest. But the ostensible point of such secrecy is to protect the country and its citizens. Here, though, it's different. The alleged conspiracy of those with security clearances is to keep from the citizens knowledge of a continuing alien assault on the human species. If extraterrestrials really were abducting millions of us, it would be much more than a matter of national security. It would impact the security of all human beings everywhere on Earth. Given such stakes, is it plausible that no one with real knowledge and evidence, in nearly 200 nations, would blow the whistle, speak out and side with the humans rather than the aliens?

Since the end of the Cold War NASA has been flailing about, trying to find missions that justify its existence—particularly a good reason for humans in space. If the Earth were being visited daily by hostile aliens, wouldn't NASA leap on this opportunity to augment is funding? And if an alien invasion were in progress, why would the Air Force, traditionally led by pilots, step back from manned spaceflight and launch all its payloads on unmanned boosters?

Consider the former Strategic Defense Initiative Organization, in charge of "Star Wars." It's fallen on hard times now [in 1996], particularly its objective of basing defenses in space. Its name and perspective have been demoted. It's the Ballistic Missile Defense Organization these days. It no longer even reports directly the Secretary of Defense. The inability of such technology to protect the United States against a massive attack by nuclear-armed missiles is manifest. But wouldn't we want at least to attempt deployment of defenses in space if we were facing an alien invasion?

The Department of Defense, like similar ministries in every nation, thrives on enemies, real or imagined. It is implausible in the extreme that the existence of such an adversary would be suppressed by the very organization that would most benefit from its presence. The entire post-Cold War posture of the military and civilian space programs of the United States (and other nations) speaks powerfully against the idea that there are aliens among us—unless, of course, the news is also being kept from those who plan the national defense. [No, please don't take the bait dangling in that last sentence…]

Tuesday, February 8, 2011

A Brief History of Observing the Sun

A little history of sun-watching and science from our friends at the Solar Dynamics Observatory.

Source: Universe Today

Monday, February 7, 2011

Definisi Planet - Mengapa Pluto tidak termasuk kategori Planet?

Mungkin beberapa tahun lalu, jumlah planet yang kita kenal ada 9, yaitu: Merkurius, Venus, Bumi, Mars, Jupiter, Saturnus, Uranus, Neptunus dan Pluto. Namun, tahukah Anda bahwa pada tahun 2006, International Astronomical Union (IAU) telah menentukan definisi planet yang baru. Imbas dari definisi planet yang baru ini sangat besar, karena Pluto yang sudah familiar dikenal sebagai sebuah planet akhirnya harus tersingkir dari "gelar"-nya. Tahukah Anda mengapa Pluto akhirnya "tersingkir"?

Planet, secara etimologis berarti pelancong (wanderer). Pada akhir abad ke-19, istilah Planet sudah menjadi istilah umum, meskipun belum ada batasan yang jelas mengenai kriteria suatu benda yang dapat dianggap sebagai planet. Umumnya, istilah "planet" diberikan kepada objek yang mengitari Matahari dan berukuran lebih besar daripada Pluto.

Setelah tahun 1992, astronomer telah menemukan banyak objek di luar orbit Neptunus (dikenal dengan istilah Trans-Neptunian Objects atau TNO) dan ratusan objek yang mengitari bintang lain (extrasolar planet, lihat artikel sebelumnya). Penemuan ini tidak hanya menambah jumlah dr objek yang potensial disebut planet, tetapi juga memperluas kenaekaragaman dan keanehan (peculiarity) dari objek-objek yang "masuk" kategori planet berdasarkan definisi/pengertian umum. Beberapa objek yang ditemukan tersebut ada yang lebih kecil daripada satelit Bumi, Bulan dan ada juga yang cukup besar untuk menjadi sebuah bintang. Penemuan - penemuan inilah yang membuat astronom merasa adanya kebutuhan untuk menentukan definisi dari sebuah Planet secara jelas agar tidak sembarang objek bisa dianggap sebagai planet.

File:Outersolarsystem objectpositions labels comp.png
Plot of the positions of all known Kuiper belt objects (green), set against the outer planets (blue)

Perlunya definisi yang jelas untuk Planet menjadi semakin jelas ketika ditemukannya TNO yang diberi nama Eris. Ukuran Eris lebih besar daripada ukuran Pluto, yang sebelumnya dianggap sebagai ukuran minimum untuk sebuah planet. Oleh sebab itu, pada bulan Agustus 2006, International Astronomical Union (IAU) mengadakan konferensi untuk membuat definisi baru Planet.
Eris as seen with the Hubble Space Telescope


The final definition, as passed on 24 August 2006 under the Resolution 5A of the 26th General Assembly is:

Illustration of the outcome of the vote

The IAU resolves that planets and other bodies, except satellites, in our Solar System be defined into three distinct categories in the following way:

(1) A planet [1] is a celestial body that:
  • (a) is in orbit around the Sun,
  • (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  • (c) has cleared the neighbourhood around its orbit.
(2) A "dwarf planet" is a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects [3], except satellites, orbiting the Sun shall be referred to collectively as "Small Solar System Bodies".

[1] The eight planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
[2] An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
[3] These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

The IAU further resolves:
Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects[1].

[1] An IAU process will be established to select a name for this category.
The IAU also resolved that "planets and dwarf planets are two distinct classes of objects", meaning that dwarf planets, despite their name, would not be considered planets
Penjelasan di atas adalah bunyi dari keputusan IAU mengenai definisi Planet yang baru. Secara sederhana, syarat- syarat sebuah objek dapat dikategorikan sebagai planet dalam tata surya ini jika:
  1. mengitari Matahari
  2. memiliki massa yang cukup untuk mencapai kondisi kesetimbangan hidrostatis (secara sederhana, objek yang sudah mencapai kondisi kesetimbangan hidrostatis memiliki bentuk bola sempurna).
  3. telah "membersihkan objek-objek tetangga" dari orbitnya. atau dengan kata lain, massa Planet adalah massa yang dominan dibandingkan massa seluruh benda lain yang berada di orbit yang sama.
Sebuah objek yang tidak termasuk satelit dan hanya memenuhi dua syarat pertama akan diklasifikasikan sebagai dwarf planet (planet kerdil). Bagi objek yang hanya memenuhi syarat pertama (mengitari Matahari), akan disebut Small Solar System Body (SSSB) atau objek kecil di tata surya. Draft awal merencanakan akan memasukkan dwarf planet sebagai sub-kategori dari planet, tetapi karena keputusan ini akan mengakibatkan penambahan beberapa lusin objek ke dalam tata surya, draft ini dibatalkan. Di tahun 2006, yang termasuk dwarf planet adalah Ceres, Eris, Makemake, dan Pluto. Keputusan ini termasuk keputusan yang kontroversial dan menimbulkan dukungan dan kritik dari cukup banyak astronom, tetapi definisi inilah yang dipakai resmi hingga saat ini.

Jad, berdasarkan definisi yang baru ini, saat ini ada 8 planet yang diakui (Merkurius, Venus, Bumi, Mars, Jupiter, Saturnus, Uranus, dan Neptunus) dan ada lima planet kerdil (Pluto, Ceres, Eris, Makemake, dan Haumea). Definisi ini hanya berlaku untuk Tata Surya kita karena extrasolar Planet belum diketahui dengan jelas/akurat ukurannya. Extrasolar planets atau exoplanets akan didefinisikan dalam referensi lain, yang memisahkan/membedakan exoplanet dan dwarf stars (bintang kerdil).

Pertanyaan evaluasi:
1. Mengapa perlu adanya definisi baru untuk Planet?
2. Jelaskan definisi baru/syarat-syarat sebuah objek dijadikan Planet? Planet Kerdil?

Friday, February 4, 2011

Mendeteksi dan Menemukan Extrasolar Planet

Belakangan ini banyak dibahas di berbagai media tentang penemuan planet di tata surya lain. Dalam artikel ini, akan dibahas beberapa teknik 'sederhana' yang digunakan astronom untuk menemukan planet di luar tata surya.

Seperti yang Anda ketahui bahwa bintang akan selalu terlihat sebagai point of light (sumber titik cahaya) meskipun menggunakan teleskop (kecuali untuk beberapa bintang yang besar dan 'dekat' dengan kita). Oleh sebab itu, dapat diperkirakan bahwa mengamati planet yang ada di bintang lain tentunya bukan perkara yang mudah.

Sebelum kita membahas bagaimana menemukan planet extrasolar (planet yang ada di luar tata surya kita), akan dibahas terlebih dahulu sekilas mengenai proses pembentukan planet.


Semuanya berawal dari material awan debu. Tata surya (planetary system/sistem keplanetan) berasal dari awan berputar yang maha besar. Awan kabut itu (nebulae) mengerut di bawah gaya berat diri, membentuk piringan dengan protosurya yang sangat padat di pusat. Akibat pengerutan gravitasi suhu naik di dalam awan (pengerutan Kelvin Helmholtz). Di pusat kian sangat panas, lalu terpicu reaksi bom nuklir, dan pengerutan piringan akan berhenti.

Planet-planet terbentuk oleh akresi planetesimal dan akumulasi gas di dalam kabut surya. Planetesimal di tahap awal tatasurya, tabrakan dan akresi (saling menempel) membentuk protoplanet. Planet dari unsur-unsur berat terbentuk dan memadat di bagian dalam, suhu jadi lebih panas (di pusat), unsur-unsur ringan berdifusi ke tepi luar. Proses itu dikenal sebagai diferensiasi dari unsur-unsur.

Bintang yang masih muda (yang terbentuk di pusat akresi) tiba-tiba menyemburkan tenaga kuat, tenaga jet dan sangat singkat, dan membersihkan tata surya dari materi pembentuk planet yang tersisa. Bintang-bintang muda penyembur tenaga semacam itu dikenal sebagai Bintang-Bintang T Tauri .

Setelah itu, tata surya akan 'stabil'. Planet - planet butuh jutaan tahun untuk menggumpal dan membersihkan 'orbit'-nya serta mendingin hingga mencapai kondisi stabil.


Para astronom telah menemukan planet-planet mengorbit di bintang-bintang. Planet besar, seperti Yupiter, menarik bintang pusatnya ke dalam sehingga bintang terputar dalam satu orbit kecil mengitari titik pusat massa mereka. Planet yang mengorbit bintang lain itu disebut extrasolar planets.

Meski Planet sangat besar, tetap tak bisa dilihat, karena bintang sentral sangat terang. Namun, pergerakan kecil yang ditempuh bintang sentral karena gravitasi oleh planet, kadangkala dapat terdeteksi. Para astronom mengukur dengan teliti pergerakan bintang dengan memperhatikan sinarnya. Sinar bintang itu bergantian bergeser ke riak gelombang merah dan ke riak gelombang biru. Telah terdeteksi dengan cara itu lebih dari 100 extrasolar planet. Cara itu dikenal sebagai metode Pergeseran Doppler.

Beberapa planet yang sudah ditemukan:

OGLE-2005-BLG-390Lb planet extrasolar terkecil saat ini (2006). 188 extrasolar planet (18 April 2006) berbagai rentang massa dan periode orbit, namun planet sebesar massa Neptunus sangat sedikit/belum terdeteksi pada jarak > 0,15 SA dari bintang pusat. OGLE-2005-BLG-390Lb bermassa 5,5 (-2,7 to +5,5) massa bumi. Pada jarak pisah 2,6(-0,6 to +1,5) SA dari bintang kerdil-M bermassa 0,22(-0,11 to +0,21) massa matahari (68% rentang kepastian). Teori akresi planet meramalkan banyak planet bermassa lebih kecil daripada planet Neptunus ditemukan daripada planet raksasa Jupiter.

Jadi, ada banyak metode yang dapat digunakan oleh astronom untuk mendeteksi keberadaan planet/sistem keplanetan di bintang -bintang lain. Metode-metode tersebut antara lain:
  1. Kecepatan radial (pergeseran Doppler)
  2. Astrometri (proper motion, sangat terbatas)
  3. Gravitasi Mikrolensa (planet dan bintang induk berada di depan bintang latar belakang)
  4. Metode Transit (planet lewat di depan bintang induk)
  5. Piringan Circumbintang (distorsi awan debu oleh planet yang mengorbit)
  6. Pengamatan Direct (langsung) oleh teropong Spitzer.


Jika astrometri langsung mengamati bintang, maka metode KR, mengamati gerak bintang dari spektrum cahaya. Yakni secara sistematik memperhatikan pergeseran garis spektrum serapan dan pancaran. Dengan teleskop sekarang, hanya dapat diukur kecepatan sedikitnya 3 m/s. Bumi, misalnya hanya mempengaruhi gerak matahari sebesar 0.1 m/s. Dengan mengukur T dan mendapatkan massa bintang, mBINTANG, bisa ditemukan 1/2 sumbu panjang orbit.

Jika massa bintang dapat diturunkan dari (mis. Diagram H-R) dan inklinasi orbit terhadap bidang ekliptika, i, diketahui, maka massa planet, mP dapat dihitung dengan persamaan di bawah ini. Jika i tidak dapat diketahui, maka yang kita peroleh hanyalah mP sin i.

Jadi sekarang kita sudah dapat menghitung massa planet (bila mengetahui inklinasi atau dengan mengambil asumsi).

Kecepatan radial untuk beberapa planet:
u/ Jupiter: v = 13 m/detik dan periode T = 12 tahun.
u/ Bumi : v = 0.09 m/detik dan periode T = 1 tahun
Limit deteksi hanya 3 m/detik, jadi planet-planet semacam Bumi sangat sulit teramati.

Penemuan pertama extrasolar planet terjadi di tahun 1995 di bintang 51 Pegasus.
Kini, lebih dari 120 planet seukuran Yupiter telah ditemukan di bintang-bintang lain dengan metode KR/Doppler. Orbit-orbitnya pendek, eksentrisitas tinggi serta harga massa mencapai setinggi 10 massa Yupiter.


Pertanyaannya: Dapatkah keberadaan planet seperti Jupiter diketahui dengan astrometri?

Sayang sekali, belum dapat. Mengapa? Mari kita lakukan perhitungan singkat.

Matahari mengorbit pusat gravitasi Matahari-Yupiter pada jejari orbit hanya 1.2 jejari matahari. 1.2 jejari matahari memetakan sudut sebesar 5.2 x 10-3 detikbusur pada jarak 1 parsec – atau 5.2 x 10-4 detikbusur pada jarak 10 pc. Kecermatan pengukuran hingga sudut sekecil itu masih belum dapat (sulit) dilakukan.

3. METODE MIKROLENSA (memanfaatkan sifat/fenomena gravitational lensing)

Gravitasi Mikrolensa terjadi jika planet dan bintang induk berada di depan bintang latar belakang.


Saat sebuah planet (benda gelap) melintas di depan bintang induknya, sebagian sinar bintang induknya akan terhalangi (ter-gerhana-i) oleh planet yang melintas. Peristiwa ini disebut transit planet (lihat diagram di bawah ini). Astronom akan mencari bintang2 yang kecerlangannya menurun secara periodik.

Jika sebuah bintang jauh di transit oleh sebuah planet semacam Yupiter, terjadi penurunan flux sinar sebesar 1% di bintang itu dari semulanya.

Sebuah planet yang telah ditemukan di bintang HD209458 dengan metode KR; pada tahun 1999, diamati kembali flux bintangnya. Ditemukan transit tepat pada waktu yang telah diramal sebelumnya. Seperti planet di 51Peg, planet itu besar dan mengorbit dekat sekali dengan bintang – planet semacam ini dikenali sebagai “hot Jupiters”.

Metode transit inilah yang digunakan oleh Teleskop Keppler. Teleskop ini dikhususkan untuk 'mencari' planet serupa Bumi. (penjelasan lebih detailnya silakan lihat di sini dan sini). Hasil kerja teleskop ini dapat dibaca pada link yang diberikan.


  • Metode KR hanya dapat mendeteksi planet-planet masif (sedikitnya 1/5 massa Yupiter) dengan periode relatif yang sangat pendek.
  • Kebanyakan planet-planet yang terdeteksi berada sangat dekat dengan bintang (kurang dari ~0.1SA)
  • 3-4% bintang-bintang serupa matahari memiliki planet-planet jenis itu
  • Sejumlah kecil planet-planet yang lebih jauh umumnya mempunyai orbit yang lebih eksentrik (e >~0.2)
Planet-planet yang sudah ditemukan beserta informasi massa bintang induk dan periode orbitnya.

Bintang - bintang di angkasa ini sangat banyak. Bagaimana astronom dapat memilih bintang mana yang diamati/dicurigai memiliki sistem planet?

Sistem planet tidak bisa terbentuk pada bintang bintang yang luminositasnya besar. Hal ini disebabkan bintang-bintang seperti ini memiliki massa hidup yang cenderung singkat. Bintang-bintang generasi I (yang terbentuk dari material Big Bang) juga tidak mungkin memiliki sistem keplanetan karena kurangnya unsur-unsur berat. Jadi, bintang-bintang yang mungkin memiliki sistem keplanetan adalah bintang-bintang yang tidak terlalu panas dan termasuk Generasi ke dua atau lebih (bintang yang materialnya berasal dari sisa material bintang lain yang meledak lewat Supernova/hembusan saat pembentukan Planetary Nebulae).


1. Materi pelatihan olimpiade Astronomi oleh Tim Astronomi ITB
2. Wikipedia

notes: jika ingin melihat gambar yang ada dengan lebih jelas, silakan klik di masing-masing gambar