The origins of the Earth

Read more about it here. Interesting.

Baby Planet Formation

captured inside Saturn's rings. Read about it here and here. It is so reassuring when you actually see the things the computer models tell you should be taking place.

My take on the definition a planet

As you know (and if you didn't, read the comments on the post below describing Pluto), but there is a legitimate debate whether or not the definition of a planet should be dynamical (orbital parameters, mass, etc.) or geophysical (essentially, is it spherical and orbit a star?) in origin. Personally, I prefer the dynamical view, and find the geophysical view too inclusive. Sphericity means its mass is above a certain value, or that the mergers that produced the object were at sufficient velocity to melt the product such that is cooled into a spherical shape. The number of spherical objects in the Solar System is large, and as a result personally don't feel that should be the primary definition of what makes a planet. It doesn't bother me that Ceres is not taught in schools because, frankly, it isn't that special. It is simply the largest asteroid in the asteroid belt - it is not even the only spherical asteroid.

I prefer the dynamical view because it better incorporates our current understanding of star and planet formation. A cloud of gas and dust gravitationally collapses into a star. As it does so, a thin disk of gas and dust form around the star. Inside this disk, collision between dust particles result in proto-planets, which then coalesce through collisions into planets. These objects are sufficiently massive that, through gravitational interaction, they most "clear" their surroundings of gas and dust - opening a gap in the disk (which has been observed in several systems). The friction inside this disk is believed to lead to circular orbits, but this doesn't have to be the case - as recent observations of exoplanets indicate (as you point out). This does mean that the objects orbit in the same plane.

If one was to define a planet based on this model for planet formation - which may or may not be the best thing to do - the requirements would be:

(a) "By far" the most massive object in its vicinity, a signature of the object clearing out a gap in the proto-planetary disk in its formation. By this criterion, Ceres would not be a planet because it is located inside the asteroid belt, whose total mass is larger than that of Ceres. The same is true for Pluto. The mass of known objects in orbits similar to that of Pluto is greater than that of Pluto. Note this criterion does not demand sphericity, just that the object is much more massive than the sum of all objects in similar orbits.

(b) The object orbits the same plane as other planets in this system. Now, this is a little tricky formally because it is circular - an object is a planet because it has an orbit like other planets. However, in our Solar System, the orbital plane of Mercury through Neptune varies by only a few degrees. [This is true for objects in the asteroid belt as well, which is the asteroid belt is often called a "failed planet" - it would have merged into a planet if not for its location near Jupiter]. Pluto's orbit, and that of the other massive spherical KBOs, is far off the ecliptic.

This definition is far from perfect, but I personally feel it better represents our current understanding of planet formation. Admittedly this is not the official IAU definition, but the official IAU definition incorporates these two elements. This definition is also general enough that it incorporates many of the "odder" exoplanets you mention - the ones with high eccentricity, the giant planets very close to their star, the planet that orbits its star backwards. As more exoplanets are detected, and these planetary systems and our own Solar System are better studied, the definition will and should change again. The new IAU definition I believe is a step in the right direction, and Dr. Stern's is a step in the wrong one - I feel that too many objects satisfy this criteria for this designation to have any meaning. This view is shared by a vast majority of astronomers I know (including the ones who study exoplanets and Solar System objects), and I believe that of a majority of astronomers - though far from unanimous as you point out.

And yes, dwarf planet is not a great name. Plutoid is better. I do feel that looking for self-consistency in astronomy nomenclature is futile, because frankly there is none.

Lastly, it is not clear that Pluto satisfies the dynamical (Dr. Stern's - quoted in the comments below) definition of a planet being "any non-self-luminous spheroidal body in orbit around a star." The center of mass of the Pluto - Charon system is outside of Pluto. Also, the gas giant planets radiate more light then they receive from the Sun. Therefore, they could be considered self-luminous, and would also not be considered planets under this definition.

[For those reading the comments below - Hydrostatic equilibrium is when, everywhere inside an object, gravity is balanced by an outward force. This is true for all stable objects (essentially by definition) - spherical or not.]

What do you think? What is your opinion? Does this make sense? Let me know, I'm genuinely curious.

A Planet around a Brown dwarf?

If so, this is very puzzling because Brown Dwarves have masses only a little bit bigger than the most massive planets? Does this mean that, in some scenarios, planets can form like stars? Go here to read more about this puzzling discovery.

Description of December 31st Radio Show: Latest Astronomy News

Long available here, below is a description of the December 31st episode of this radio show, where I discussed the major Astronomical results to come out in December 2006. They were:

• Solar System News: Recent powerful solar flare produced both neutral and charged particles, first time neutral particles detected from such an event; ESA's Venus Express detects hydrogen leaving Venus's atmosphere from the day-side for the first time - gas appears to be result of water being broken down into hydrogen and oxygen in upper atmosphere, and might explain why there is so little water on Venus; NASA's THEMIS satellites detect the opening of a large hole in the Earth's magnetic field due to the interaction with a clump in the solar wind; NASA instrument on India's Chandrayaan-1 Lunar spacecraft measures changes in chemical composition of lunar surface on small physical scales (link, image); next NASA mission to the Moon, the Lunar Reconnaissance Oribter; completes major milestone before April 2009 launch (website); evidence for climate change on Mars resulting from regular changes in the tilt of its orbit around the Sun - deduced from studying patterns of rock layers in a large crater (link); results from ESA's Mars Express satellite suggests that peculiar light-toned deposits result on ground water bursts onto the surface (link); NASA's Mars Reconnaissance Orbiter completes primary mission (website), releases new 3D images of surface (link), and found evidence for carbonite on Martian surface which requires the past presence of neutral or basic water on Mars's surface (article); evidence that site of NASA's Phoenix Mars Lander is currently going through a "dry" phase; next NASA mission to Mars - the Mars Science Laboratory - delayed until late 2011 due to cost over-runs, delay expected to have implications for future planetary missions; ongoing debate whether future joint NASA/ESA Solar System mission should be to Jupiter's moon Europa or Saturn's moon Titan; recent flybys of Saturn's moon Enceladus by Cassini show continuing activity and of Titan show evidence for volcanos spewing ice in to this moon's atmosphere; Swift satellite detects X-ray emission from Solar System comets (link)
• Wednesday Morning Astronomer: Yes, an ice cream carton hurtling towards the Earth at 99% the speed of light would be a very bad thing (article); no, the astronomical theory for what Voorwerp might be is not an attempt to upstage the Dutch schoolteacher who found it, but is an attempt to understand a still very bizarre object (link); star formation in the universe is actually on a decline, and if the current thinking on the universe is correct, the ultimate fate is a starless night sky - a very depressing prospect indeed (link); and I think it is definitely a good thing that the more we look, the less we understand (link).
• Calendar of upcoming Astronomy/Science events in the greater Poughkeepsie/New York area.
• More News: Evidence that brown dwarfs form like stars not planets (link); up and coming sub-mm radio interferometer ALMA gets its first telescope; astronomers discover the two faintest brown dwarfs known (link); analog to young Sun identified by CoRoT satellite (link); turbulence in proto-planetary disk important in determining how planets form (link); Spitzer images massive stars destroying proto-planetary disks of their neighbors (link); Dutch undergrads discover first extrasolar planet orbiting a fast-rotating star (link); Hubble Space Telescope discovers carbon dioxide in the atmosphere of an extrasolar planet (link) and measure the size of a different extrasolar planet (link); proposal to look for moons around extrasolar planets through the "wobbles" they create in their orbits (link); new technique proposal to look for water in the atmosphere of an extrasolar planet; direct imaging of extrasolar planets is Science magazine's #2 breakthrough of the year; Kepler spacecraft - which will search for Earth-like planets around Sun-like stars - shipped to Florida (just launched last week!!!)

As always, please post below or email any questions, comments, or concerns you might have. Thank you for listening!

October 15th Radio Show: Star Formation, Planet Formation, and Massive Stars

Continuing my recap of topics I discussed this summer, here is the October 15th episode of the radio show, where I discuss the latest results concerning star and planet formation, massive stars, in addition to the usual Astronomy news and calendar. I'll post a more detailed description later, but in the mean time, enjoy! Comments and concerns are always appreciated, and please leave any you have below.

Description of August 13th Radio Show: Planet Formation and Extrasolar Planets

Below is the detailed description of August 13th edition of this radio show, already available here. On this program, I talked about:

• News: No, Phoenix Mars Lander has not detected life of Mars, even though one of the instruments on the Phoenix Mars Lander did detect a type of salt called perchlorate - a fuel source for some micro-organisms on Earth (link); ice in Martian soil may be preventing soil samples from entering the instruments on Phoenix Mars Lander; Cassini prepares to fly within 30 miles of geysers erupting on the surface of Saturn's moon Enceladus (link), initial data already received; Hubble Space Telescope celebrates its 100,000th orbit around the Earth by taking this image of a star forming region near star cluster NGC 2074 (link); instruments on the Hubble Space Telescope slated for repair during final servicing mission to occur in October (link) - astronauts interviewed August 11th; new book on how the Hubble Space Telescope got built recently published - "The Universe in a Mirror" by Robert Zimmerman; Interstellar Boundary Explorer mission undergoing testing of science instruments prior to its October 5th launch; bizarre object discovered by Dutch school teacher Hanny van Arkel in archived images of the night sky maintained by http://www.galaxyzoo.org (link); David Dunlap Observatory in Toronto sold to a developer; NASA's Aerospace Safety Advisory Panel releases annual report, available here; NASA readjusts time table of Constellation Program - the successor to the space shuttle; NASA awards prized at General Aviation Technology Challenge (link); NASA says farewell to this year's group of high school and college aged INSPIRE interns (link to INSPIRE program); NASA awards space radiobiology grants (link); Arizona students talked with astronaut Greg Chamitoff on International Space Station on August 15th.
• "Wednesday Morning Astronomers": An astronomer's opinion on the Astronomy content in Gregg Easterbrook's weekly column on ESPN's Page 2, available here - yes, it is extremely cheesy that Doritos broadcast an ad towards a nearby star, and I also hope they aren't offended by it. I also find it extremely disturbing that a NASA press release concerning a Spitzer observation of a star similar to that of the Sun was quashed for being "alarmist."
• Calendar of upcoming astronomical and science events in the greater Poughkeepsie/New York City area.
• Planet Formation: As explained previously no this program by Dr. Aki Roberge, planets are believed to form in the gas and dust disks left behind after a star forms. The starlight from the central star has a large effect on the gas and dust in this disk, possibly driving larger icy bodies out to the solar system before gas drag causes to circulate inwards. This can lead to the formation of organic molecules and water in this "proto-planetary" disk, as was recently observed (article). Planets in this disk are expected to grow from dust grains colliding and sticking together to form pebbles, which in turn stick together to from proto-planets, which then form planets. These dust grains and pebbles scatter the light from the central star, and this scattered light has been observed from the proto-planetary disk around KH 15B binary star system. As the planet grows, it carves out a gap in the disk detectable in radio observations of the gas and dust. This has been seen around a very young star, <100000 years old, HL Tau - implying that planets form very rapidly (link), as well as around older star AB Aurigae (link).
• Extra-Solar Planets: Currently, there are three major observational techniques to detect planets around other stars - the radial velocity method, which detects the motion of the central star caused by the gravitational pull of the planet, the transit method - which detects a decrease in the light from the central star caused by the planet passing between the Earth and the star, and microlensing technique - where the gravitational field of the planet bends the light from the star such that its brightness on Earth increases very quickly. All of these techniques require monitoring the properties of many stars at once. An upcoming project to do this using the radial velocity method is MARVELS. The radial velocity method requires precision measurements of the star's spectrum since the planet's signature is a periodic change in the central frequency of spectral lines. This requires very good calibration of the spectrometer, which might be easier and 100x more sensitive thanks to a new laser standard (link). There are many transiting planet searches being conducted on the ground and from space. One such satellite, the COROT mission, has discovered a planet orbiting a Sun-like star, though at an orbit very different from the Earth's (link). NASA has also directed a spacecraft which orbits comets to look for transiting planets in it's spare time, the EPOXI mission (link). A new detector possibly capable of detecting the transit from an Earth-size planet around another star is being developed in the UK called "RISE" (link). The microlensing technique has recently been used to detect a planet just three times the size of the Earth, the smallest extra-solar planet yet. SuperWASP, a transiting planet survey, has recently detecting 10 extra-solar planets with different sizes and orbits (link). A rocky planet has been inferred around another star due to its gravitational effect on a previous detected planet in this "solar system" - it has since been detected using the radial velocity method (link), as have three other Earth-like planets (link). Small planets have also been detected around small stars like brown dwarfs (link). It has been speculated the nearest star to the Sun, Alpha Centauri, might have Earth-like planets around it (link). Last, but not least, even though planetary nebulae have nothing to do with planets, a planet has been discovered inside one (link).

Sorry for the delay in posting this. Thank you very much for listening, hope you enjoyed it, and any and all feedback will be appreciated.

August 13th Radio Show: Planet Formation and Extrasolar Planets

Now available here is the August 13th episode of this radio show. In addition to the usual Astronomy news and notes, and the return of "Wednesday Morning Astronomer", I talked about the latest news and results concerning our understanding of how planets form and the properties of planets detected orbiting other stars. I hope to put a more detailed description online, soon. I promise.