How long can the most extreme planetary systems survive? Measuring the tidal orbital decay of hot Jupiters.

几个世纪以来，人类一直梦想着地球以外的行星，甚至太阳系以外的行星。1995年，随着第一颗围绕着另一颗类似太阳的恒星运行的行星的发现，这些梦想开始成为现实。这颗系外行星之所以引人注目，是因为它看起来与我们所熟悉的任何行星都非常不同。这颗行星是一个巨大的气体巨星，类似于木星，但只需4天就能完成其恒星的一周公转(相比之下，距离太阳最近的行星水星需要88天，木星需要近12年)。从那时起，发现了更多这样的行星，其中一些甚至离它们的恒星更近(纪录保持者的轨道周期只有18小时)。这些系外行星被称为“热木星”。关于这些短周期的热木星，我们面临的最大问题之一是，它们离主星(比地球离太阳近50倍甚至更多)还能存活多久。在这些距离上，行星和恒星之间有强大的潮汐作用力(很像地球和月球之间的作用力)。这些力会导致行星失去传递给恒星的能量。行星轨道的能量损失导致行星的轨道逐渐变小，行星慢慢向恒星盘旋，直到它被摧毁。我们不知道的是潮汐轨道衰变的速度有多快。这可能发生得相对较快，热木星的寿命只有几百万年或几千万年，不到恒星寿命的百分之一。或者，这种缓慢的死亡之舞可能需要数十亿年，所以一颗炽热的木星的预期寿命将类似于它的恒星。这个时间尺度由一个量Q*决定，它告诉我们恒星从行星轨道吸收能量的效率有多高。不幸的是，我们不知道Q*有多大；事实上，估计值差异很大，这就是导致热木星寿命存在巨大不确定性的原因。这个项目旨在准确测量几个行星系统的Q*。我们将使用从地球上看直接从其宿主恒星(凌日)前面经过的系外行星。正常情况下，我们希望观测到的这些凌日完全是以规则的间隔间隔的，因为行星每轨道运行一次。然而，如果轨道正在经历潮汐衰变，我们预计会看到凌日变得越来越近。据预测，在几年的时间里，这种影响只会发生几秒钟的变化。然而，有了现代望远镜、照相机和先进的数据简化和建模技术，这是可能的。测量Q*不仅可以告诉我们热木星的预期寿命，还可以告诉我们恒星和行星之间潮汐相互作用所产生的其他过程的时间尺度。例如，这将帮助我们确定，当我们认为热木星一定是在更远的地方形成的，而那里的温度更低，那么热木星是如何在离它们的主星这么近的地方存在的。

For centuries, humanity has dreamed of planets beyond the Earth, and even beyond our Solar System. In 1995, these dreams started to become a reality, with the discovery of the first planet orbiting another star like our Sun. This exoplanet was remarkable because it looked very different to anything we were used to. The planet is a massive gas giant, similar to Jupiter, but takes just 4 days to complete an orbit of its star (contrast this with 88 days for Mercury, the closest planet to the Sun, and almost 12 years for Jupiter). Since then, many more such planets have been discovered, some even closer to their star (the record holder has an orbital period of just 18 hours). These exoplanets are known as 'hot Jupiters'.One of the biggest questions we have about these short-period hot Jupiters is how long they can survive so close to their host star (50 or more times closer than Earth is to the Sun). At these distances there are strong tidal forces at work between the planet and the star (rather like those in action between the Earth and the Moon). These forces cause the planet to lose energy which is transferred to the star. This loss of energy from the planetary orbit causes the planet's orbit to get gradually smaller, and the planet slowly spiral towards the star until it is destroyed. What we don't know is how fast this process of tidal orbital decay is. It might be that this happens relatively rapidly, with hot Jupiters only living for a few million or tens of millions of years, which is less than one percent of the star's lifetime. Alternatively, this slow dance of death might take billions of years, and so the life expectancy of a hot Jupiter would be similar to that of its star. This timescale is determined by a quantity Q*, which tells us how efficient a star is at absorbing the energy from a planet's orbit. Unfortunately, we don't know how big Q* is; in fact estimates vary hugely, which is what causes the enormous uncertainty in hot Jupiter lifetimes.This project aims to accurately measure Q* for several planetary systems. We will use exoplanets that pass directly in front of their host stars (transit) as viewed from Earth. Normally, we expect to observe these transits spaced at exactly regular intervals as the planet transits once per orbit. If the orbit is undergoing tidal decay, however, we expect to see the transits getting ever-so-slightly closer together. The effect is predicted to be just a few seconds shift, measured over a period of several years. This is, however, possible with modern telescopes, cameras and advanced data reduction and modelling techniques.Measuring Q* will not only tell us the life expectancy of hot Jupiters, it will also tell us about the timescales of other processes resulting from tidal interactions between star and planet. This will, for example, help us to determine how the hot Jupiters came to exist so close to their host stars when we think they must have formed further away, where it is cooler.