Searching for Earth’s Twin

December 22, 2011

This has been a great year for experimental big science with ground-breaking findings in particle physics and astronomy. One of the most remarkable breakthroughs has been the success in the search for planets around other stars. The word exoplanet which first appeared in print in 1995 according to google, and has become a popular term in news reports in just the last three, has become ever more familiar this year as reports from the Kepler space telescope have taken the number of candidate exoplanets into the thousands.

Kepler is constantly watching 145,000 main sequence stars in our nearby region of the Milky Way galaxy in the direction of the constellations Lyra and Cygnus. It is looking for the tiny dimming of light that tells us that a planet has passed in front of the stars disk. By recording the amount of dimming,  how long it lasts for and how frequently it repeats, Kepler can estimate the size and orbit of the planet. In February NASA released a catalog of 1236 candidate exoplanets and this month the number increased to 2326. These have to be verified by ground based observation and so far the catalog of confirmed exoplanets has 716 entries.

The real interest about exoplanets concerns whether or not there is other life in the universe, and if there is, how common it is. A whole new industry of exoplanetary statistics has been born with scientists inventing habitability indexes that can be applied to the exoplanet catalogs to gauge which ones could support life. One habitable planet catalog has two exoplanets regarded as more Earth-like than Mars. These are HD 85512b and Gleise 581d, both found earlier this year, but they are rather large to be comfortable for us to live on. If they have an atmosphere it is could be too thick due to the stronger gravity. Already Venus has a thick atmosphere making the pressure too hot and high pressure for us to survive. If we discovered an exoplanet like Venus we would be very excited because it is in the habitable zone and is very similar to Earth in size. Finding out about its atmosphere would be difficult from a distance of many light-years.

This week some new “Earth-twins” were announced Kepler 20f and Kepler 20e. They are very similar in size to earth but they are not in their Suns habitable zone where the temperature would be about right for liquid water and conditions similar to Earth. It is good news that Kepler has proven that it can find planets of this size but we will need to wait before we find ones where we could really live. It is said that these planets may have been further from their star in the past so that life could have formed there in the past. This just serves to emphasize one more characteristic an exoplanet must have if it is likely to support life as we know it on Earth. It must stay in a stable orbit around a stable star for billions of years so that life can evolve without being obliterated by heat or freezing.

Kepler has a planned lifespan of 3.5 years and may have its life extended. This should give it time to find some more earth-like planets orbiting Sun-like stars with periods of about 1 year. Kepler takes time to find these because they need to pass in front of their star at least twice to confirm their existence and orbit. because Kepler works by looking at such transit events it only sees planetary systems whose disk is aligned with Earth. If it were looking at earth from afar it would have only a one in 700 chance that this alignment occurred. If Kepler finds  one Earth-like planet we could guess that there are 700 in the sample they are looking at, which represents 1 millionth of the stars in our galaxy, but will it really find any?

My guess at this stage is that it will find a number of Earth-sized planets and a number of small planets in the habitable zone, but the statistics may be against it finding an Earth-sized planet in the habitable zone of a stable star like ours. Probably we will be able to estimate how many such planets there are and it may be something like a million in our galaxy. It could be a lot less. The next step will be tp determine how many are likely to have the right chemical mixture to form water and an Earth-like atmosphere. We don’t yet know the answer, but it is exciting that the data we need to answer these questions is starting to become available.

How Earthlike are Kepler’s Latest Exoplanets?

February 3, 2011

I am sure everyone is aware of the latest release of exoplanet data from Kepler that has multiplied the number of known exoplanet candidates by a factor of about five. Kepler detects its exoplanets by looking for stellar transits so it is only going to see them in the rare cases where we are in alignment with the plane of the stars planetary system. Luckily it can look at a lot of stars in a patch of the sky all at the same time. In its first few months it has found well over a thousand by this method. Some of these may prove to be glitches and must be verified either by land-based observations of by repeat transits observed from Kepler.

So which is the most Earthlike  planets they have seen? To answer this you need to peruse the full set of data which can be found here. Even then the answer depends on what you consider to be the most important parameters to define an Earthlike planet. After due consideration I am going to go for Keplar-268 which has an estimated radius of 1.75 times the Earth, a year of 110 days and it sits at 0.41 astronomical units from its parent star. This should give it an estimated surface temperature of 295 degree Kelvin or 22 degrees Celcius. Admittedly it is a bit large so its gravity is going to be stronger than  we would probably enjoy.

The estimated temperature that NASA uses is based on the amount of received radiation. I’m not sure if there is any correction for greenhouse effects which depend on the density and content of its unknown atmosphere. In any case it is at least reasonable to assume that its rotation will not be locked to its star so it has a chance of being habitable with liquid water present. On the other had it’s high gravity may mean it retains too much atmosphere and suffers from permanent clouds making its surface very hot and high pressured.

This is just the first big release of data from Kepler and more can be expected, especially since many Earthlike planets will not have done a full revolution of their star in the time it has been l0oking. The results so far suggest that when all data is collected there should be some candidates for really Earthlike planets, at least in terms of size and ambient temperature. Once their location is known it will be the job of other telescopes to look at them in more detail. This will include the best Earth-based telescopes using adaptive optics and interferometry to focus in on the systems. A little later the James Webb Space Telescope should take over, if and when it successfully reaches its position to start observing in space.

First Earth-Like Planet Discovered!

January 15, 2011

So How often do we think we are going to see the headline announcing the discovery of the “First Earth-Like Planet” ? The latest example (CoRoT-7 b) came just as the New Year arrived. It turned out to be a firey world with temperatures ranging between -210 to 2000 degrees centigrade because it is tidally locked with its star. That’s not what most of us would consider Earth-Like. It earned its dubious title by being a rocky planet not very different in size than Earth.

Even as the news broke the sense of deja-vu was overwhelmingly strong. The previous “First Earth-like planet” (Gliese 581g) had been discovered just four months previously. This one is in the Goldilocks zone of its star meaning that it is at the right distance for liquid water to form on the planet. However, water would only actually form if it was rotating to give the surface an even temperature, but this one is probably tidal locked too.

Looking back through the news archives it is no surprise to find that news of the first Earth-like planets first appeared as far back as 1998 when just a handful of exoplanets were known. since then there have been a number of candidates, hopefully with each one being a little bit more Earth-like than its predecessor, although it is also the case that some of these discoveries later turn out to be errors.

At viXra log we confidently predict that the next collection of such headlines will hit us when the next major update from the Keplar mission is released on February the first, if not before. The pre-release of info about CoRoT-7 b may however signal that nothing much better has been found. In any case we can be sure that planets described as more and more Earth-like will be appearing for many years to come.

So what do we really think should be counted as an Earth-like planet? The data we get from Keplar and other observations should tell us about the planet’s size, mass, and distance from its star. From this information we can infer its average temperature, whether it is likely to be tidally locked, the strength of its surface gravity. Then from this we can make a guess about whether it could support liquid oceans of water, and an atmosphere of the right density as well as whether it has a molten iron-rich core. the latter is important because it could give the planet a magnetic field that protects it from radiation.

If “Earth-like” means a planet could support an ecosystem like ours it will need to be in the Goldilocks zone of a star that is not too dim so that it does not tidally lock and liquid water can exist. It will also need to have a size and density reasonably close to Earth’s. That will ensure that it can retain its atmosphere without it becoming too dense. It also means that large land based animals will not be hindered by excess gravity and the molten core will not solidify to remove the magnetic field as it did on Mars.

It follows form these considerations that the star around which the planet orbits must also be like our Sol. If it were less bright the planet would need to get nearer and would be tidally locked. Bigger stars tend not to last long enough to provide a stable environment in which life can evolve. Other types of star may be too variable in brightness, or may have regular deadly flares that could strip any nearby planet of its atmosphere, even with the protection of a magnetic field.

Taking these things into consideration, the parameters that we can currently measure of exoplanets and the stars they orbit need to be very close to those of Earth and our Sun.

Apart from these considerations, an important feature of our planet is its large moon. Without it there would be no tides and these have surely been very important in promoting the evolution of live. The Moon is also said to keep our planet’s rotation axis stable. If it were otherwise we might suffer catastrophic changes in climate that would ruin the atmosphere of our planet. There may be an outside chance that Kepler could see the presence of such a moon as the system transits in front of its star.

Even if a star meets these requirements it really just makes it potentially Earth-like. The planet also needs to have the right chemical mix of elements in the right quantities to make oceans and an atmosphere that would support rather than poison life. Will we ever be able to detect their presence?

After Keplar has made a long (hopefully) list of potentially Earth-like exoplanets the next step will be to examine them more closely using other observatories. The best hope of being able to do this in the foreseeable future lies with the James Webb Space Telescope that is due for launch in 2014. The JWST will have spectrometers sensitive enough to see the slight differences in a systems spectra when planets pass behind their stars. It will even have a special camera with a spot that can block out the light from a star so that some planets can be seen away from the glare. This should certainly be good enough to work for the gas giants but it may require the next generation of space telescope to be able to do the same for smaller (potentially) Earth-like planets.

In the long term the prospects for finding truly Earth-like planets can only get better. Just how fast depends on technological and economic developments that are hard to predict. Ultimately there is no reason why we should not be able to determine the chemical composition of the atmospheres of Earth-sized exoplanets and if they have the right proportions of oxygen and nitrogen we will know that Earth-like plant and animal life must be present. Perhaps we will even be able to see the tale-tale signature of chlorophyl or other molecules that can only be produced in quantity by vegetation. That’s  if such planets even exist nearby.

In any case one thing is for sure:  Headlines telling us of the “First Earth-like Planets” are going to be around for some time.

Most Earth-Like Exoplanet Found by Keck

September 30, 2010

Astronomers from UCSC using the Keck observatory in  Hawaii have announced the discovery of a planet in another solar system that orbits in the Goldilocks Zone of its star. The star Gliese 581 was previously known to have five planets but this new one could be hospitable to life “We had planets on both sides of the habitable zone – one too hot and one too cold – and now we have one in the middle that’s just right,” said Dr Vogt, quoted by the BBC.

However life there may not be easy. The rotation is believed to be locked to the star so that only a small region in the perpetual twilight zone is likely to be suitable for life. Anything living there would have to be careful not to stray into the hot zone facing the Sun or the cold zone in eternal shadow. There may also be problems brought on by the planet’s gravity.  It is four times more massive than Earth, so assuming the same density it might have 50% stronger gravity at the surface. That could mean a heavier atmosphere. Nevertheless there is a small chance that the atmosphere and everything else  is also just right and there is a possibility for life to evolve and survive there.